(The reply form suggests me to consider really good if I want to revive this topic - and I think I want because I have just today thought of a totally working and QS-friendly way to reset the odometer to a non-zero value. So, just for posterity, if anyone comes searching.)
For example, after having finished a book (say on page 200), I want to start another one, where the real text I beemind reading starts only on page 15.
My data points, consecutively, will be:
200
0
-14
0
14
And here I am, with a goal ready to receive any further progress, without any not-earned buffer.
Unlike retroratcheting suggested above, this keeps the total number of pages read QS-true.