I too am having trouble determining what you mean.
I assume that by ”vertical” you refer to the orientation of the wheel in the stand.
Which would mean that you’re working on getting the wheel round.
Now, all trueing stands I’ve ever seen have been adjustable for different wheel sizes.
I can’t imagine not being able to align the gauge to the brake track, or whichever part of the rim you use to reference off.
Unless there’s a considerable error in how the the stand is assembled
Also, are the gauges ALWAYS off the rim, or do they move off the rim as you spin the wheel?
It is entirely possible to get a wheel out of round. But if it’s laced properly, and have the right spoke length, that really limits how far out of round you can get it.
There is some variation in how builders approach true and round.
Last wheels I built, the rims were so round that my usual incremental trueing basically brought them into round as well, without any special effort.
Other rims, particularly single-wall rims, it can require quite some effort to bring them into round.
Doing too many turns of the nipples in one go can easily result in a seriously egg-shaped or oval wheel.