B series cam timing
On a B-series the cams gears are 100mm in Dia. So the circumfrence of the cam gear is 314mm (100mmxPI). Divide 360 degree by 314 = 1.14 degree per mm.
So for every 1mm milled the cam gears will roll back (retard) 1.14 degrees at the cam or 2.3 degrees at the crank.
So you have a 0.020 milled meaning 0.50mm = about 1.14 degrees retarded, so advance your gears 1 degree to compensate for the 0.020" mill.