From my experience with SFI I will tell you getting active affiliates it's what concerns me and not best country. Generalising your advert is advisable than to be looking for best countries to advertise. Sometime I look at the first 100 most active countries and pick the first 10 countries to advertise or buy PSAs from there. still, it's all bunch of inactive PSAs but surprisingly I got 1 PSA from a country that is not among the first 100 and she is very active.
So in my own opinion it is not how far but it is how well. Generally, everywhere you can get active PSAs is your best country. I have seen advert on ECA of PSAs from UK, USA, Canada with a very costly price. The question is if I get PSAs from these countries at that price can I force them to be active? Or would they be active? I don't think so because I have a lot of affiliates from these countries that are inactive. Yes someone may recommend such countries because of their good economy but that it's not a guarantee to SFI business. But the power of MOTIVATIONS and good LEADERSHIP guarantee your success in SFI.
My answers to your question if I may be honest with you. Generalise your advert globally. See you at the top.
From my experience with SFI I will tell you getting active affiliates it's what concerns me and not best country. Generalising your advert is advisable than to be looking for best countries to advertise. Sometime I look at the first 100 most active countries and pick the first 10 countries to advertise or buy PSAs from there. still, it's all bunch of inactive PSAs but surprisingly I got 1 PSA from a country that is not among the first 100 and she is very active.