Try this with the scope:
Put both channels on DC coupling and set up the scope to display (CH1 - CH2) in YT mode.
Measure the same signal and get the difference zeroed out on the scope.
Change only CH2 to AC coupling, which removes the DC component.
Now your difference signal should be the DC component only!
You can use the PCB adjustment to minimize the difference signal on the scope.
And if you really need a number, measure the difference with the cursors.
(But you don't really need a number.)
Why would I do all of that when I can just clip a DMM to the XY test points, and adjust the value to zero?
AC coupling is safer for the scope. Especially when you're working with things that can output large amounts of DC without you knowing.
Things should be made as simple as possible, but not simpler.
When you have an "asymmetrical" waveform like typical X or Y vector signals of most games, the "DC" value your meter will give you is "average" value of the waveform. This is NOT the true DC offset of the waveform (from the standpoint of setting the offset of a BIPolar DAC output). You might want to use the scope on DC, where it should normally be (IMHO), to verify where you are setting those pots.
I understand what you're saying about the offset of the BIP. But we're just looking for a ballpark check of the DC component to make sure it's in the range that is safe for the monitor before connecting it. So the average value is what we want here. Any DC component could be coming from whatever the board is drawing at the moment (correct OR incorrect), plus any offset. But what the monitor cares about (in terms of not blowing it up) is the average value. We're not trying to get it perfect, but rather just make sure we don't nuke the monitor, and that we have it in a place to get a decent starting point image on the screen.
The ideal way to optimize the pots is by adjusting the image on the monitor screen (i.e., what they are meant for), which you still need to do once you get everything hooked up.
Looking at a number also gives you a feel for what's 'right', and when a number isn't right, it's easy to spot. For example, when adjusting a Tempest, I want to see the DC value rise up to about 3.5V briefly when the attract mode is drawing the scrolling TEMPEST graphic, then come back down. And I know that when it's in test mode, the DC should be around -1V, because the test mode result screen draws more things on the bottom of the screen than the top. The DMM gives you plenty of information. (And again, I'm trying to provide info here for people that doesn't require them to have a scope.)
Any procedures I describe here were refined over years of working on these boards, in practical conditions. There's a method to my madness. I also worked in high-speed T&M for 20 years, and I currently work at Keysight, so I know scopes. But I'm not going to go full-blown measurement geek here, when my main goal is trying to help people fix their games with the simplest equipment possible.