How far can one safely deck a 225 block?

So, I have this question. Why Std bore? The cylinders probably have wear. and be out of round. You are thinking about increasing the CR by decking the block, and maybe milling the head. You might have a problem with ring seal. Without making sure the cyls are round and straight, keep your CR reasonable.

EDIT: Question #2. Zero increase. All slant six engine (USA Automotive) had flat top pistons.