I would be also interested what would be tennis countries ranking list. (pro tennis is much more than just top 100 players )
FIDE has similar ranking list for chess.( country rank by average ranking POINTS of top 10 players) Very interesting to compare if my country is better or worse than neighbours.
Estonia has five ATP-ranked players:
In total they have 718 points.
If I gave the Estonian top-10 71,8 points on average, would that be what you had in mind? Because I can do a nationwise calculation in this fashion if you want. It would not be very much like the chess ranking, though. Roger Federer alone will beat most nations, something you would not see in chess. A decent chess amateur has way more than half as many points as Magnus Carlsen, whereas an equivalent tennis amateur does not have points at all.
Do you have another idea for how this nation ranking could be calculated?