As an aside, Grant - If you weld yourself a triangle -a big one if you
need it- you will have a "Planar Jig" this can then be used to clamp
other (more than 3 joints) so that they lie on the same plane.
Just be sure your joints are fairly flush on the jig's corners.
As far as cutting a round tube on the same plane, I have also used
"gravity" to line up the pipe. Just clamp a heavy weight to the middle
of the pipe - (supported on the other end by something too) and then
once the pipe has stopped wobbling - clamp it down. Good for +/-
joints. For really accurate joins I use a v-block.

Tom