Yes, I was wrong with the colour coding indeed (green shades is what GC uses).
To me the most useful scale would be the percentage of your own finds. If you have 2000 finds in total, and 300 of them in Nirvanistan, this country would get the 15% colour. Absolute numbers might also work but then small countries would never get out of the low % ranks while percentage of area covered would have the same issue for large countries, with the added disadvantage of requiring added calculation. But technically all these should be possible.