From whats been posted on both of our Opel forums in the past, the cam timing retards 2 degrees for every .025" milled from the head and/or block. So .050" milled would be 4 degrees. One way I've been told to restore the cam timing is to drill a new cam dowl hole in the cam gear. Since there are 3 cam bolts, there are 3 different orientations the gear could be bolted on to the cam, if the dowl wasn't there. It sounds simple enough to just index the cam gear 120 degrees plus the amount to be advanced and have a new close tolerance clearance hole for the dowl, which is 6 mm, if you can find someone with the right size drill and reamer.
I think I said that right. You should probably wait for a more definate answer first. Evidently there are a couple more ways to accomplish this, but which is the simplest, I don't know first hand.
You shuld also ask the experts how much compression increase you'll get from this.
HTH
Paul