In Gen 5 the chance of getting a shiny using the Masuda method is still a massive 1/366. (But it is a lot better compared to Gen 4 masuda method which was 1639/1.) Compared to the usual rate of 1/8192, it is amazing. You have only gotten 400 + squiltes, if you get over 1366, then you have a problem.
Source: Bulbapedia