i have been messing around with this more and more. 
Here are some of my thoughts Istra if you are working on an addendum...
You really should add in the selection options 
Shift to Add, 
Ctrl to Subtract and 
Shift+Ctrl for Intersection. That would be some good advice, but I think you should explore using the 
Quick Mask. This allows for for a lot more editing options.
Just some thoughts!