if you have a quick look at the split polygon tool option box you will see a 'magnet' (or something similar) slider. This slider states how much the split jumps to either end of the edge, or its centre.

So a high number will see the split jump to the center of the edge (making a 90 degree cut), while a low number will give you more control over the location (on the edge) of the spilt.

Hope this helps :p