Joe,
I have found that the lower scribe line--that which demarks (or did originally) the top of the bottom paint--is a good reference for level. The boot top scribe (the upper line) is
not an accurate line for level, as this scribe is substantially sheered. The boot top is never a good reference for anything, including painting a new boot top.
If you remove the paint, you should be able to see the scribe marks at both ends, as well as at the centerline. These scribes survive very well and are difficult to sand away even if you want to. Leveling fore and aft can be as simple as measuring (with a plumb line) from the scribe mark to the floor and ensuring that the measurement is the same bow and stern--assuming the floor is a flat, even surface like a paved driveway or concrete floor. I wouldn't do this on gravel.
This is getting ahead of things a bit, but the boat is properly trimmed when she floats parallel to that lower scribe line, regardless of whether the boat is heavy (floats low) or light (floats high). Most Tritons tend to squat aft and really should be trimmed forward to properly even out the trim so that she floats
parallel to that scribe line. Properly painting the boat in this manner makes quite a difference in the overall appearance. The original waterlines require raising almost as a matter of course, as I've yet to see a Triton with original waterline location that looked right. It took me 2 seasons with the boat in the water to figure this out, after which I corrected the waterline and boot top, as detailed
here. Do not repaint the bottom to the top of the old boot top scribe as a means of raising the waterline; it will never look right. If raising the waterline, the new line must be struck parallel to the original lower scribe, and the boat trimmed to suit this line rather than the other way around. (2 vastly different Tritons--
Glissando and the Daysailor--both float parallel to this line, though with about 6" of difference in height between the two.)
FWIW, I think sailboats always look better if trimmed slightly by the bow rather than the stern (if not level). Squatting looks terrible in all cases, but level is best. My eye always goes directly to the waterline and boot top on all boats that I see; bad ones are common, and immediately detract from the boat's appearance regardless of how nice the other details might be. This is one of those pesky details that makes some boats look right, and others less so, even to untrained observers, though they can't necessarily put their finger on why one looks better than the other.
For side-to-side leveling, running a long level across the cockpit, from sidedeck edge to sidedeck edge, is as good as anything. You can also pick a consistent point on the lower scribe line amidships to level the boat side-to-side. It's worth taking the time and effort to level the boat at the onset of a project, as you then have accurate level and plumb lines from which to build your new structures and interior.
Here's one archaic way to level the hull.
Leveling the Daysailor
I'll use a good transit/laser level to level future boats.