[Donnchadh MacGarry]

so I conclude with a beginner suggestion from beginners: don't trust too much a pre-built setup of a very experienced driver, because you're probably going to have a worse performance than the one you get from your safe, "stability first", setup.
I'm trying to work on that for now, and I humbly suggest you do the same,

I pretty much agree with this bit, as a beginner you do not need to focus on setups at all and in fact downloading ones by someone who is setting records on RSR or whatever might be 

1. Undriveable for a beginner
   
2. Undriveable for a well experienced racer over the course of a race. They can only do 1 hotlap and that's after many attempts where they constantly restart the run (they never seem to show that part in the video, I wonder why).
   

Getting a setup that you are comfortable with is the most important thing. I wouldn't stress on it too much yet, judging by the time difference between and the "pro setup", I personally believe you have much more time to gain in your driving technique though improving your racing line and brake points. This is especially true when you are over 2-3 seconds slower then the fastest guys here on open setups. There are a few little things you can do to help though

• Tyres: Open the tyres app (default or ProTyres), and do 3-5 laps. Drive at a good pace, but if you are spinning then you need to restart again as you will have cooked the tyres and will get bad readings. Look at the hot tyre pressures and make adjustments to the pressures so that the hot tyre pressures will be in the ideal range. getting the tyres in the ideal pressure range helps with car stability immensely, you will really notice this if you haven't tried it before.
  
• Fuel: Do 5 laps (which you should have done for the tyre readings). you should now have a reading on the average fuel used per lap which is pretty reliable in my experience. If you want to do calculations manually, convert all time values into seconds to avoid confusion when doing the division and multiplication when determining the number of laps in a race and the fuel required. NEVER UNDERESTIMATE FUEL, always add a lap or two extra if you are not sure.
  
• Geraring: Find a final gear ratio that means you are close to the rev limiter in you last gear at the end of the longest straight (where the car should be at its fastest over the lap). Allow for a small bit of headroom, so you are still able to gain some more speed when slipstreaming someone on that straight so it makes the overtake even easier. If you hit the rev limiter long before the end, lenghten your gear ratio as this increases top speed but reduces acceleration. If you are getting nowhere near the rev limiter on the last gear, then you can shorten the gear ratio which will improve you acceleration but reduce the cars potential top speed (but in this case that doesn't matter).
  
• Brake pressures: If you are always locking up, perhaps reduce the brake pressures. If one end is always locking, you can adjust the brake bias away from that end to reduce the brake load at that end. Even now, I rarely adjust my brake settings and just adjust my own inputs instead, so I am actually still poor at knowing what to change with them.
  
•  Aero: This is only necessary for very aero dependent cars, generally open wheel cars. Tracks with long straights and low speed corners require low downforce aero packages, as they reduce drag and thus increase top speed. You will not notice the difference between a low downforce setup and a high downforce setup by too much on low speed corners, so its not a huge sacrifice there. Tracks with short straights and medium to high speed corners require high downforce packages, as they allow a driver to take more speed into these corners and losing top speed doesn't matter as much on shorter straights.With aero it is all about finding a balance, that is key. Start with a value for the front and rear wing that you think will suit the track, keep one value fixed and make changes to the other one when you want to make changes (I think usually the rear is fixed and the front is adjusted to find balance, but I could be wrong on this or it might be best for some cars to fix the front and adjust the rear for balance). Do a few laps and ask how is the car on medium to high speed corners. Too much understeer means there is not enough front downforce (or too much rear downforce), so increase the front wing (or reduce the rear wing). Too much oversteer means there is too much front downforce (or not enough rear downforce) so reduce the front wing (or increase the rear wing). It can be a good idea to create a low, medium and high downforce packages for a car and then select an appropriate one for the track you are running, but this would only be if you are using a particular car for many different tracks.
  

As I said before, I don't think you need to worry too much about setups. In my first 2-3 years of sim racing, I just focused on my driving techniques as I had far much time to gain by improving that. The only things I changed were tyres, fuel and gearing as mentioned above, I didn't even go at aero (although I only entered the fixed series when using cars that were very aero dependent). Last summer was about the first time I tried my hand at making some setup changes having spend a large amount of time doing some theory research. Even now I find it very hard to know what to do with some settings, dampers I find very hard to know what to adjust when doing some testing even though I think I have an idea on the theory. Making a good setup can only be done when you are able to drive very consistently very close to the limit, otherwise you will not know if the changes you made to the car made a difference to you lap or if it was actually the fact you happened to be driving a little bit better that time. Only when I started to be able to lap at a pace relatively near the fastest drivers and doing so very consistently lap after lap within a 1 second window for the 20 minutes session in practice (as long as the tyre wear wasn't too excessive of course), was when I started to think I was ready to move on from the list above and start looking at the other settings in the setup. Personally, I don't think there any of the default setups that are completely undriveable, certainly for the slower cars. Cars like the F1 cars or the Mazda 787b are a bit of an exception but often people tend to put up stable setups for those here in the forums. A good driver will pretty much always be able to drive any setup at decent to fast pace and more importantly do so very consistently. So until you get to that stage, then as a beginner I really don't think you need to worry about setups yet.