I don't plan to compete in this test race, or the league itself, but having read through the first page of this thread I think that you may all have missed something...
The race will end when the first car crosses the line after 6 laps (for example). If the Formula XRs have been lapped once by the leading FO8, for example, then they will only complete 5 laps. In terms of time, both classes will always have races of a similar length when they are competing in the same session.
So you can all stop trying to cater for both classes with the lap counts! If you want 20 minute races, aim for 20 minutes using the expected FO8 lap times and the FOXs won't race for much more than 20 minutes!
The GFC used to race mixed-class with FOX and FO8, check out some of their
race results to understand how LFS deals with it.