Mill & drill press tramming tool

[Toolmaker51]

Who's he calling 'old-timer'? The reason for a single indicator is finer measurement; by increments and repeatability. Legitimately, and per inspection standards, instruments used commercially are acceptable to 4x finest graduations.
So we use an .0005, good parallels to 'skip' over the Tee-slots, occasionally after sweeping a fine stone over the table for nicks.
1] Wipe clean table & parallels with solvent/ nonabrasive hand cleaner/ or water based cleaner. White paper shouldn't expose any remaining smudge.
2] Lower the table and extend the quill just short of full travel [not against the stops]. The added extension will magnify error.
3] Set parallels near front and back edge of table for nod, shift same distance apart for swivel.
3] As suggested, loosen the clamps for both trunnions [swivel & nod] just snug and sweep them simultaneously.
4] Approaching Zero in either direction, increase torque on clamp bolts as you would lug nuts across centers. This will displace the indicator, especially if clamps were too loose to attain movement and stable setting.
5] I usually finish with backing off worm screws to not bear on rack teeth fully, just touching. That way, a reasonable check in the future is when there is still minimal pressure on the worm.
This is how we used a very special genuine Bridgeport way back when. Equipped with optical scale readers in .0001's, it was our jigbore. Never used a more accurate type of reader before or since. They measure actual position, much like a vernier, instead of interpreting electronic signals. The limitation of electronics is in resolution, both have nearly same repeatability, optics have an edge in physical accuracy.
Another nice feature was a stainless rule attached at front of table. Below, a sliding tab and pointer, with about 1.5" travel, positioned toward 1/10 inch graduations. Once zero was set [scales or dials] you sailed between coordinates as fast need be by hand or rapids. Instead of counting .200, .400, .600, .800, 1.000, to 3.348 whatever, watch the rule and pointer, reading dial for final thousandths. No one can read a flashing readout faster, even counting rotations. It does require holding backlash on the same side [x+ or -] during use.

Like that?
Masking tape and fine point ink-pen will do the same thing. One on the table, one for the bed. Just count up to first setup.