Interesting question. Old school here;
Target wires hung from the overhead on piano wire with fish plate weights to steady them in the pit. A number of ways to secure the wires, but once set and secured, they’ll not move. Brackets were set (and anchored) to centerline off of these wires. Rail gage with 1” dot from the rail face was used.
Yeah two stops were easier with the plumb, but a straight piece of 3/4” conduit beveled on one end was used for the BG stick and rail facing one to the other no matter what the number of floors. I’m sure there are more “modern” ways today to achieve a plumb and faced rail job which (should) equals a smooth ride. Never used a level of any sort for rail install.