Agreed! I personally have the same problem and am quite motivated to fix this. Our planned “generalized road dial” may be part of the solution to this. http://expost.padm.us/dial
In the meantime, you can hit Retroratchet and then figure out manually what your historical rate has been. Not ideal, obviously, but does work.Kyle commented
That is what I've done.
A related feature might be the ability to track data without a goal, but then being able to turn that data into a goal later on.