||It depends on what you're happy with regarding combustion chamber volume, valve clearance, etc.
You can reach a point where you'll have to slot your timing belt tensioner to get the required ~4mm gap. If you're not willing to do that, I'd say that is your limit. I don't know how much that equates to as far as block and head machining is concerned.
The point I was trying to make is .008" is not a hard limit. I've used heads where the combined block and head machining was .012", and had no problems. 0.019 is pretty warped, however. Machining that flat will almost guarantee camshaft bores will need machining too.