Hey Jon,
Are you having fun yet? The Enfield isn't like any other modern bike I've owned either. Exact measurements and fluid levels are something you don't really need to worry too much about. The big thing is DON'T overfill it with oil. This is bad news.
Check the oil by screwing the dipstick all the way in. That's supposedly the correct method. I never fill mine to the 'full' mark, just a 1/4" down or so. My experience has been, that as long as there's oil showing on the dipstick, its just fine. You don't have to screw the dipstick in all the way, if you account for the fact that the oil won't show as high on the dipstick. Does that make sense? I try and keep the oil level right around the middle of the dipstick.
There are three drain plugs on the bottom of the crankcase. I open all three and loosen the timing bolt to let all the oil drain out. Then tighten up the drain bolts on the crankcase, leaving the timing case bolt loose. Fill up with oil. Run the engine until you see new fresh oil squirting out of the timing bolt (this verifies you have oil feed to the cylinder head and all is well). Tighten up the timing bolt and let it idle for a minute or so. Shut off the engine, allow a few minutes for the oil level to stabilize and check the oil as described above.
When I replace the oil filter, I splash it around in fresh oil prior to installing it. Insert the new filter and follow the directions above. I've never had any problems doing it this way. I'm sure other will jump in if I've said somthing misleading.
Hope that helps. If you haven't got the Pete Snidal Manual, I highly recommend you spend the money. It will tell you how to work on the Enfield, its quirks, and for the most part why you are doing it. Its good stuff. Good luck.
Paul