One little tip that I use to ensure you don't get plaster blow out on the inside is to use the core bit on the inside to just cut through that first half inch or so of plaster. Then move to the outside to cut the hole. The pilot hole will ensure that the two cuts line up perfectly, and as you have already cut through the plaster on the inside you don't get it blowing out as the core goes through. Sure, it does make a little dust on the inside but worth it to keep the plaster in one piece.