Dave99 writes:

Is this the best way to accomplish this, or is there a better way I'm
missing?

This is really two functions, cylindrical workholding versus 90-degree
indicating.

Vee blocks are one suitable workholding device for cylindrical things. Use
the clamps provided, or pinch the work against a vise jaw with the vee.

Assuming you don't have a precision rotary workholder like a rotary table
or collet holder, you can mark 90 degrees accurately one time on your lathe
chuck and thereafter hold the work in that for scribing. Or at the least
you can wrap paper around the tube and divide that as a template.