Seems fairly straightforward that the best of weekend approach could easily be 75% of the races. I really don't know how one arrives at the drop 6 races methodology. Where are the rules posted again about this?
This year its working out to be 71% because of that first weekend with 5 races. There are a total of 17 races this year so subtract the weekend with the most races in case a driver couldn't make that one. That leaves 12 races.
In a season where each weekend has the same number of races or has 5 weekends instead of 4, the percentage goes up quite a bit. If that first weekend had 4 races instead of 5, we would be at 76%. In a season with 5 events, we'd be at 80%. Hope this helps to clarify.