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To measure vertical overscan take a grid test pattern and view it on a LCD and count the bars/squares separating top from bottom, then feed the same pattern into the set and count what you see. If say there is one inch/division on screen and you are missing two evenly then you have one inch overscan on each side (top and bottom that I mean).
You should try to avoid more than 1" of image overscan as it will make the scanlines more spread out and easy to see.
Optimal configuration is to get the scan lines as thin as possible with the focus and get them close together/overlapping with the vertical size/linearity controls (by minimizing overscan). One thing that can accentuate scan lines even when overscan is at minimum is failure of the set to properly interlace on scan I think the term is "double scan" or "line pairing" or something similar (some member had a roundy with that issue years back)....
If you have a lower voltage source than the one you think is too high you can connect a 100k-1M pot at the ends to the two sources, and the center tap to the focus lead.
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