It's fairly easy to calculate EV for every move you make. The hard part is to figure out what the chances are that the opp will give you his whole stack (or any portion of it) to you. This is the thing that's giving me a headache. There are so many things to consider: opp's image, own image, stack sizes, board texture... and this is what I'm calling the X -factor.

I suppose it's the thing that comes with experience.