Originally Posted by mtber

Think about it like this...
You are making the distance between the cams and the crank different, hence the geometry is going to change due to the deck hight difference.

I think the following notes are from Endyne:

B-series cam gears are 100mm in Diameter
The circumfrence of the cam gear is 314mm (100mmxPI).
Divide 360 degree by 314 = 1.14 degree per mm.

So for every 1mm milled off the head of block the cam gears will roll back (retard) 1.14 degrees at the cam or 2.3 degrees at the crank.

0.020 milled meaning 0.50mm = about 1.14 degrees retarded, so advance your gears 1 degree to compensate for the 0.020" mill.