All drivers who complete 90% of the race laps are deemed as classified finishers in line with the official FIA classification.

However all drivers are given a ranking, and for the purpose of match and positional betting, this official ranking shall apply.

To work out if a driver has completed 90% of the track, you will need to divide the drivers completed laps by the total track laps.   

So if it was a 70-lap race and the driver completed 63 race laps, that is 90% of the race (being 70 total laps) so that driver will be deemed a 'classified finisher' of that race.