I usually start with something similar to the ruler method myself. I usually use a ruler as a feeler gauge and move the nozzle to each corner and adjust from there. If I’m starting from a brand new bed corner install, I’ll set all 4 corners to the same height adjust (5 style-ish corners) first and then double check the z axis both side with my handy dandy built in measuring tape to make sure they are both level, then heat to print temperature but retract a bit so I don’t have to deal with too much ooze. and then measure the corners. I then use my micrometer barrel z height adjust screw to fine tune the overall bed and run the 4 loabed 2 pass bed calibration pattern. from there I can see if any of my corners are two high. Then I start in on calibration cubes and whatnot.
Depending on what model printer you have, there are issues with measuring anywhere but the corners. if you measure in the middle on a 1-5 you will see an apperent “hump” due to the 10mm rods drooping. the bed rods also have this a little bit, but the bed, though heavier, si more spread out so it droops less. That makes it look like your bed is humped in the middle. A 6 has 12mm X rods so it is less prone to that, but the 10mm bed rods remain, so it sometimes presents with a little bit of a dip instead. There are mods that take care of that if its a concern to you.
Using a dial indicator is also problematic. If you bolt it to your extruder, you are now measuring with more mass than normal, if you bolt it in place of the extruder, its usually lighter than the extruder. If you take the time to make the dial gauge carriage the same mass as your extruder you can get a decent measurement, but subject to the droop issues above.