This is so simple I suspect everybody except me thought of it long ago, but I've never seen this particular technique described.
When tramming a mill, the table slots are problematic when rotating whatever indicator you're using. Sometimes people put a solid plane surface on the mill table to rotate the indicator on, but that adds to the complexity. The other day I figured out an easy way to measure directly off the table and avoid the table slot problem.
I clamped my magnetic indicator base to the spindle, ran the indicator arm out about a foot on one side, and made indicator contact with the table. I zeroed the knee elevation dial, dropped the knee a bit, raised it back to zero, then confirmed the dial indicator was zeroed. At that point I dropped the knee again so the indicator wasn't touching, rotated the spindle 180 degrees so the indicator was on the other side, and raised the knee back to zero. Simple. Even if there was a slight error in the table elevation, a thou error over a two foot diameter is generally ignorable.