Sorry, just a guess: if the tracks gdb file has entries for the race length (laps or time), the % will be adjusted to that.
I think i remember someone somewhere saying it was a percentage of the amount of laps or time usually run at events at that track, but its a very vague memory and im probably wrong, sorry that was'nt much help
@Alek I would suggest to just try it out. Take the new zandvoort 2020 track. In the gdb file of that track, the default length is set to 90 laps. Now, in the dedis options screen, set the race length to "% Track Laps". And in the race laps field enter 10 (will be shown as 10%). Leave the race time empty (N/A). If the race lasts 9 laps now, you know how it works.
