[Russell Sobie]

You can calculate easily the total laps of the race dividing the time session between the best lap time of the qualy.
For example, let's say 20 min race and a time of 1:46 for the poleman:
2000 / 146 = 13'6 laps, so you should use fuel for 14 laps to be more than safe, thinking that the race times won't be as fast as qualy times.

There is also an easily way to calcule it: download the fuel app from RaceDepartment, it will show you how many liters do you need to finish the race.

You broke all sorts of stuff in your math above.

20 minute race is 20x60 seconds, or 1200 seconds. 1:46 lap time is 60+46 or 106 second lap time. 1200/106=11.3207, so 12 or 13 to be safe. Your math comes close, and is luckily (in this case) on the safe side, but it's a teeny bit high. I've been doing it long hand like the above this whole time, and I'm actually surprised for timed races AC doesn't do the calculation for you without an app. 

I think I installed the Fuel App at one point and just forgot to enable it... so I should probably look into that.

Or you can just go here on your phone (googled for this and actually found a post here on the SRS forums): http://fuelmanager.tk/

Edit: Did some minor testing, and this web app looks like it tends to be a bit on the safe side as well. For instance, lap time of 45 seconds and a duration of 50 minutes would give my math 66.667 laps, so I'd say 68 minimum, and his app says 68+1 laps. I guess he has some variance in his algorithm to account for relatively long races with short lap times? 50 second lap times at 20 minutes I get exactly 24 laps (25 for safety) and his says 25+1 laps. Essentially I would recommend using this website. Or just do the calculations yourself. With 60 seconds in a minute, of course.