If you take the time to make a plug, then take a few more minutes to drill and tap it clear through. Then get a piece of threaded rod and make a second plug a few thousandths smaller than the bore of the through hole. On this second 'plug' you'll need a blind hole in the back of it for the threaded rod to bottom out in and then you have an adjustable solid stop that you can run all the way out to your jaws if needed. (Word of caution, careful about this hanging out the back of your lathe, it's a death trap if you accidentally start it up in high RPM's with the threaded rod hanging out unsupported!)
Oh yeah, double nut the rod hanging out the back or else it will loosen up and you're screwed accuracy wise~ We run ours like this for production parts ALL THE TIME...
EXIT 85 Manufacturing "The best custom wheels, period" (www.exit85.com)
Experts in low volume, highly complicated, one-off forged aluminum wheels