I would highly recommend using a tire pressure gage. I digital one would be best and easiest to use. it would be very easy to make an adapter . Higley makes a tap for the threads of a glow plug. If you asked, I'm sure they would also tell you what size and thread you would need. Get the appropriate die. Thread a piece of copper or brass tubing for the adapter. I just checked the size of a tire valve stem and it is very close to the appropriate size. You might even be able to use a valve stem for an adapter.

Just my .02