I'll tackle the water level question.
Yes, if water in the sight glass is at the halfway point, the boiler should be nearly half full, though it is important to remember that these narrow sight glasses do not always perform with complete reliability, so some judgement is best used in reading the water level shown. Sometimes bubbles form in the fittings just below the sight glass and interfere with the proper level of the water in the glass itself, so be aware of this possible false reading scenario. Some folks put a hair fine brass or copper wire in the sight glass to help "pop" such bubbles, but they do not always work as intended either. Still, most often the water level in the sight glass is at or very near the proper water level in the boiler itself.
However, because the sight glass is not the full height of the boiler, though it does generally represent the actual water level that is in the boiler, thus:
When the water level in the sight glass is at the top of the glass, the boiler is really only 2/3 to 3/4 full, which is actually as full as you are ever going to want it, as there does need to be a fair amount of room for the steam to form within the top of the boiler. A boiler that is too full will just force a bunch of water into your steam lines, and thus into your engine, which is a very bad thing and can actually cause damage. So you never want your boiler too full of water, and the visual top of the sight gauge is probably the limit of what you'd ever want in there!
Likewise, the bottom of the sight glass is perhaps at the 1/4 to 1/3 level in the boiler, and you'd be wise to never run the boiler any lower than that, as you can do serious damage to your boiler and yourself by letting the water level get too low. For best results and safest operation, all steaming should be done using only the middle 1/3 to 1/2 of the boiler's actual capacity.
After you've pulled the flame out from under the boiler, and let it cool down for a while, I'd wait until the pressure is below 5 psi as near as you can tell from your pressure gauge. Be sure to have some kind of container that you can catch boiling hot water in conveniently, as that is what you will be dealing with when you open that drain valve and steam and water come shooting and spitting out, and I'd be prepared to tip the boiler slightly forward so as to get the most complete drainage possible, which may well require a hot pad or good insulated glove.
As you start out, operate on the safe side of all parameters, until you get a true feel for the equipment, its capabilities and operating idiosyncrasies. Take nothing for granted, and try to learn something from every operating session, therein lies much of the fun in this pursuit!!!
Hope any of that helps!
.... And welcome to the Forum!!!