I bred about 50000+ eggs so far with about 43000 eggs using Masuda, gotten 83 shiny now. My overall rate is about 530 eggs per shiny, while in Bulbapedia they assume that using Masuda combine with the Shiny Charm increase the rate to 1/512 (which is pretty close to mine). So if you hatched 8000 eggs via Masuda i think i got aroung 14-16 shiny on average. Of coure the actual amount can vary since the actual sample on this case is quite small