Hello, I am restoring a 1920 Stanley Steamer. I need to have new brake drums cast out of ductile iron for it. Finished they are 14" OD X 12" ID with a half in thick web. I am going to make the pattern and I know about shrinkage but I don't know how much I need to leave for machining. I was thinking a quarter inch all the way around. And help would be appreciated.