Some great replies - I really appreciate people taking time out and posting.
I was remiss in not explaining the root problem that instigated my post. We have 9 different 18" pies - from plain cheese to the Tony Soprano which has 3 meats on it. Five of the 9 make up 70% of the slice orders, so when we have 2 slices left of those, we make another whole pie for slices. But the other 4 pies don't sell quite as well, so in order to keep them "fresher" we do half and halves - like half plain and half Hawaiian. What we don't want is one half to be larger than the other half because it effects the slice sizes (obviously). Finding the exact center of an 18" pie is not so easy when done with the naked eye - and you don't realize it until it's too late.
I think I came up with a simple fix with alligator clips and string - I'm able to do this because we start the pies off on screens. Sure, I can (usually) get an 18" pie off a wooden peel and maintain (mostly) a perfect circle, but it's hard to do everytime and almost impossible for the guys I'm training - so screens are part of the process. Probably nothing pisses me off more than getting a "small" slice off a badly cut/dressed pie - because I know for every small slice their is a partner "big" slice that someone bought for the same price.
All to say, I want to make perfect slices whether it's from a whole Tony Soprano or a half Bianco half wild mushroom.
Also on my list is this item - http://www.dvice.com/archives/2009/08/multi_blade_piz.php
Hopefully that will make my quest for 8 perfect and identical slices a consistent reality.