Not sure on that calculation. C-F-M has a calculator to use when you are looking at their Cometic's head gasket. not sure what all the numbers should be for a stock block and or head. Stock compression is 10.2:1. 0.50 is a large (think most are 20-30 thousandths) but not sure if you would get a full extra number on the left side.
I think at 0.050 you also are making the Zetec an interference type motor. I have asked before but dont recall ever getting an actual hard number of if you remove X material.
I think you will want a tune with that large of a change of compression also.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)