I did this using my premium account on project-GC for my region
using the Map compare with filter set to:
Cache location: Belgium / Liège
Profile name: Pepegeo
Show none: on
Show one: on
Show both:on
Owned=found: on
and get the total of find (577) and not yet find (2054) for my region Liege/Belgium
then the same with custom filter i made (need premium account on project-GC)
find (15) and not yet find (63)
proportion for my region is then (15+63)/(577+2054) = ~3%
you could run this search for a complete country and compare to overview for this country
me in belgium 2466 premium for ~24900 enabled is about 10%
Hope way of working help , still help