Sure, right or wrong this is how I did it:
I didn't have a dial test indicator at the time so I turned a piece of stock to .750" about 3 inches long. Near the end I drilled a hole sized for the adjustable arm off my mag base, & drilled and tapped a hole into the end for a set screw to hold the arm tight.
Mount that with a .0005" dial indicator in a 3/4" collet & the indicator vertical. I did this with the vise off. It gets the indicator ~5 inches ish off centre of the spindle. Come down with the spindle for just enough preload on the indicator for a good reading but not enough to have to mess with it dropping into the tslots.
Then I just turned the spindle, by hand, from the top to not influence the indicator and grabbed side to side readings off the same slot of the table and also corner to corner. I didn't notice much if any nod front to back so I loosened the 3 swivel bolts for the head just enough that I could get it to move with a deadblow on the side of the housing.
Get it almost perfect, repeat measurements, tighten a bit, rinse and repeat as it took a few tries and I probably need to do it again as I've moved the machine a couple times & when using my face mill it's definitely cutting a thou or two more on the right side of the cutter vs the left.
I will probably try it off 123 blocks this time with the vise on & see how that goes if everything clears.
I'll take a photo when I get back outside of the setup, it's nothing fancy but it's what I made when I bought my craftex mill