I use subtract left and right, if you just hide any brush that is not going to get cut you will be fine. Keep an eye on the readout to see how many brushes were made from the subtract so that you can see if it did what you wanted it to do(if you cut a brush and it makes 12 fragments out of it you will want to think about not useing subtract for example). I learned how subtract could hurt my map or work for me after only a few minutes of messing with it. If people just spent some time messing around with it in a box map they would have no problem with it IMO.
I put the new in b.