Congrats. I'm happy to share what knowledge I can. My manual is a copy from another TV collector
...Cut-Off. This control is used to determine if the tube under test will cut off with the proper grid voltage.
To check the cutoff, turn the Selector Switch to the CUTOFF position. Rotate the cutoff control until the meter reads at the "Cutoff" line on the meter. If the Cutoff control pointer reads within the "good" range, the tube cut-off is good.
If not, the tube may still be usable but you may have trouble with the contrast, brightness, etc.
- Bob