I like many others do most of my notches (tube fish-mouth) using a chop saw, even though I have a "hole saw notcher" and plasma cutter. However, I pretty much guess at how it needs to be cut. Sometimes it works, sometimes I grind a while, sometimes I fill a 1/4+ gap... Recently on PBB there was a thread about how this is done more precisely. The best post was by "tigweld" on PBB and is quoted below. After the quote is a link to the whole thread. I thought this was just too good not to post here (and make it easier for me to keep up with it too!) Full thread on PBB.