Enfields are a tough one to locate a level and square spot to put the ol bubble level onto. I've tried the charger bridge top, it turned out to be square on one rifle and not on others. I've put a level across the top rear portion of the bolt channel, as well as standing on the flat at the bottom of that area, both turned out to be different on different rifles. Ditto with the barrel flat by the knox form.
In the end I used a stiff barrel cleaner rod of 1/4" stainless, slid a clear plastic tube over it in two spots so I could center it in the barrel once on the rifle, (although I imagine rolling even paper tape around the rod or dowel to build it up would be sufficient too) and put the bubble on its protruding ends, that gave me a fore and aft level.
A vertical line has less reference points but end the end the most reliable method I use is a small weight on a cotton thread, set it up dead center at the top of the butt stock so the thread hangs past the butt plate screws. I usually twist the screws first so the slots are vertically in line with a steel rule, and then when the rifle is vertical the thread will hang exactly down the center of both the screw head slots. (potential drawback is obvious but so far this has been reliable)
Good onya for squaring up the sight, this is an oft forgotten part of using an aperture sight.

ps I haven't heard of using simple green for cleaning, do you use that as a degreaser or something else?