Tilted plane used for alignment

[[Se...]]

Hi all, 

Think I'm having a brain loss moment and cannot figure out how to align a particular part (will add photos when I’m in work).

Part is 1” square tube bent into a half ring, the half ring is then twisted/tilted 30 degrees, meaning there is no ‘flat’ surface to create a plane on.

Can’t help but feel I’m missing something simple?