There are 120 subway stations (on this map) on the MBTA, I've been to 37.5% of them! To find out your percentage, simply click on each station you have been to and see how you do.
There are a few caveats...this wonderful map does not include the Orange Line stop Assembly Station and it does not include the Silver Line. When calculating your percentage, I pretend that those stations do not exist. In the future, I may add the MBTA's official map, but to start I decided to use this more simple one.