Seems to me, using the spindles, because they're adjustable, as a reference point, would not be a good idea.
I've built frames, back in the 70s, and it seems to me, without putting the frame in the original fixture, finding reference points is going to be tough. You just can't assume that anything is straight.
My frames were flat across the bottom, you might call the manufacturer and ask if the frame is the same as mine.
One idea comes to me, a 3 point mounting. 2 points under the rear bearing hangers and 1 point in the middle of the front frame tube. Of course this has to be on a very flat surface. Measure the distance from the bottom of the spindle mounts to the top of the table. Measuring from the front to the rear is a good suggestion.
I would follow any instructions given to me by the manufacturer of your frame. Don't forget to independently measure the angle on the spindles. The manufacturer should be able to tell you what that should be.
Just an aside; if you can't put the frame back in the original fixture, it's gonna be tough no matter what else you do.