Or a positioning vise!
Bill........If you have the GRS turntable it should have come with a round rod to help you. If not, it doesn't matter as you can do it without. It only takes a little longer. Use one of the big holes in the vise to centre on.
Yes, with a loose scope or stand it will bump out of position easily and then you spend a lot of mucking about constantly readjusting. Get it right and lock it down...........after it's locked down, for fine tuning, I just thump the stand with my hand. Crude but it works