In the service manuals that I've seen, the procedure for setting G2 is roughly: put brightness/contrast to zero or 50% (it varies?), set the 'service mode' switch to collapse the vertical scan, and then adjust G2 until the horizontal line is just visible. Then adjust the R/G/B drive until the line is pure white in colour.

I've never replaced a flyback, but when I increase G2 on an old monitor I do it at full brightness and contrast, increase G2 until the horizontal edges of the raster are visible, and then back off G2 to just before the raster becomes visible. Funny enough even though the service manuals often seem to talk about a service mode switch, I've never actually seen such a switch inside a VGA/SVGA monitor.

I set G2 to the point just below where the retrace lines disappear when brightness is at max

I basically do what maxtherabbit does, but I use a dark room to verify that screen is blank. G2/Screen aka acceleration voltage determines the initial acceleration speed of the electrons off the cathodes into anode (the screen itself), the higher it is the narrower the beam and the better focus you can get but at cost of video amps having harder time reaching cutoff on the cathodes as they must be driven harder to stop producing electrons which is why retrace lines appear or when tinting happens at black and dark things when it is too high. Higher acceleration also produces a brighter image since there's more energy in the electrons to pass to the screen and to some extent there's a geometry shift observable too.
Normal procedures try to make sure that no voltage in the tube itself is too high and that everything stays in the capability of the video amps to avoid any problems in future when the tube is sufficiently worn.

Thanks for the input guys. I set the screen voltage as you mentioned and then found this little utility to run some tests:

https://github.com/unclejed613/video-test-bench-dos-freeware

Everything seems to be working fine!