Looking to replace my OEM Icon AirRail with something new.

Do you measure from the center of the blade to the "invisible line" coming off the dropout or from the leading or trailing edge?

Also -- how much difference does a couple millimeters make, say from 43mm to 45?

last -- will carbon dropouts get chewed to bits if placed in a velo-vise type roof rack?

Thanks. Got a bad case of upgrade-itis.

see Wim's post here on how to measure rake & cautionary about confirming length:
http://forums.roadbikereview.com/showthread.php?t=102340

No opinion about rack mounting..

A number of posters have suggested that most of us would be hard pressed to feel the difference of a few mm...

Gotcha. The graphic was helpful!