Is there a science to measuring backspacing on wheels?

[Thorpe]

Is there any real special way to measure the backspacing on wheels? I want to get some fresh wheels for my 250es, and need to measure... From the mounting surface to the outer most part of the wheel, or the mounting surface to where the bead auctually seats on the wheel?

[cr480r]

Wheels halfs are measured from mounting surface to the bead seat.. heres a couple examples an 8" wide wheel that is 3+5 offset has a 3" inner half and 5" outer half for 1.5" offset... an 8" wide wheel could also be a 4+4 which would be centered or no offset...