How far is my timing off?

[Big Yin]

The back story
I had the cylinder head of a 1995 E320 straight six machined to remove a bow from end to end after overheating.

My machinist checked his online data base and warned me that getting this particular head flat would require machining very close to the recommended limit of material removal. I asked him what impact that would have? "It is going to change the timing," was his reply.

Finally the Question!
How much does machining the cylinder head alter the timing? Is there a direct ratio? (X degrees per 0.001 inches of drop in the head relative to the block kind of thing?)
How sensitive an issue is this for this particular motor? None at all? Disastrous?

Is there a corresponding rule (of thumb, or otherwise) for offsetting the impact of machining the cylinder head? Can it be (should it be) compensated for by changing the relationship of camshaft and crankshaft gear teeth (e.g. reduce/increase number of links between gears)?

Has anyone successfully dealt with this issue? Or even know for a FACT that it is/is not an issue?

Many Thanks as always,