End Stop Position - where to place Origins

Is there any standard or anything at all that tells you where to place the origin point in an alignment?

Say you have a flat edge that's say 1,200mm. Naturally you'd put the origin at 600mm but is there anything that informs you to place it half way down the edge?

I'm getting different results as it's not a perpendicular edge (it should be). What would everybody else do?

Cheers, Ben