if your shop won't tare then check how it is wrapped on the bolt and ask that it be evened up before cutting.
This is especially true of wide backing. Cutting loses at least 8-12 inches. My local shop says they allow a bit of extra but it never compensates for the loss when squaring up the backing.
When I cut strips, I will tare the end and line it up before cutting. This can more easily be done after washing and then iron it straight.