RMS question

I am the new owner of an 88 XJ that needs a new rms. I know that I need a brass drift to remove the old one, but my question is about drift size. I have found them from 1/8 all the way up to 3/4". I'm thinking that the 1/8 or 3/16 is probably what I want but would appreciate any input you might have before I spend any $. TIA

I couldn't find a brass one.. I ended up useing a steel one and being really careful..

When you get your new seal, look at the size of the metal insert- get the drift closest to it's size?