Squaring my Y axis to the X axis

I have a shopmaster Eldorado Bridgemill that I have had for years, and don't use enough.

I was trying to make some brackets for a PnP machine I am building, and had a lot of trouble making them square. The spindle is square to X and Y, the problem is Y is not square to X.

The Y cross slide has the ability to turn +-45 degrees like a swivel vise. when I put my dial gauge in the spindle and try to set Y to 0 degrees, it doesn't seem to work.
My theory is because the spindle is directly over the pivot point, large changes in the Y angle make only small changes to my gauge .

In explaining this it a solution comes to mind - trying to measure farther from the pivot point. going to the edge of the T-slot table for example. Although I think I need to go to the T-slot because that is where the vise will be mounted. There are only 2 T-slots, and they probably less than 2 inches from the pivot point.

Can someone explain to me a procedure that will let me get Y 90.0000000 degrees from X?

Brynn

I square mine with a dial gauge mounted to the table and run that across the face of the lathe chuck by moving the Y axis.