slant 6 head shave

Doing a little more math I came across this article. http://www.moparmusclemagazine.com/techarticles/mopp_0602_chrysler_slant_six_engine/index.html

It claims they cut .268 off the head to get approximatly 13:1. Setting up a simple proportion yielded the following; for 10.5 you must cut .216. SOunds like a lot to me.