When you measure to the back of the insert and totally disregard what's going on out in front, as long as the weight doesn't change the balance point doesn't change. For setting up broadhead arrows though, I far prefer the old fashioned method that measures the total length of the arrow, from nock to tip. With that method, the balance point does shift, and it shifts in the wrong direction.
I wanted to comment on this point. It doesn't matter where I measure from. The mark I place on the arrow is the same balance point whether I use a field tip or a broadhead. On a flexing shaft, the actual balance point on the shaft is what is important. If the tip weight remains the same, it doesn't matter what the configuration is. The shaft will still flex the same and balance in the same spot.