When dimensioning a non-X Y set we have to start by selecting a line perpendicular to the desired dimensions.
I would love to see all dimension tools gain the "aligned to ref" per this idea http://forums.autodesk.com/t5/inventor-ideas/custom-alignment-direction-for-aligned-dimensions/idi-p...
However, the Ordinate Set tool could "snap" perpendicular to starting line in addition to the current "snap" aligned to starting line.
This would solve 99% of my use of non-X Y ordinate set issues and would be both easy to implement and would sit comfortably with a future "align to ref" that should be added to all dimension tools
Cheers
JF