I tried to look for the value on gc.com but i cant find it there.
The reason for the differens is that pgc caclulation works the same as gc.com worked.
You took all the FP and divided by the number of premium finds and that is how gc.com and pgc does it.
You shall only take FP that has been given by members that are premium today and divide bye the number of premium loggs. the reason is probibly that the precent value shall alwyas be 100 and below
When i looked at the users that have favourd your cache I found that the user doemis( se link below) is not premium today and sould not be included
http://www.geocaching.com/profile/?guid=0f8667b7-85d5-4f15-b909-d613c400b4c3
21 finds and 5 basic is 16 premium finds and 11 FP is 11/16=68.75% And that is still incorrect
After som test and cacluclation i foung that i was almost corretc. All FP shall be include and users that have given a FP and is a member today shall be counted as a premium member
Then we have 21-5+1= 17 "Premium finds" and 12 FP =12/17=70.588% and that is the same as the pgc value 70.59 that can be read in the vpgs.
That is the metod that is used. It gives a value 100%.