Yes to what everyone has said.
We calculate the number of units you would need to screen, with average luck, and start with that. If we have bad luck and don't find enough, we recalculate and look some more. If we find extra units, we celebrate our good luck and buy a Powerball ticket on the way home.
Our LIS doesn't really have a convenient way of making a digital worksheet, so we have a paper worksheet to record our results, and to copy from to enter results on the units in the LIS. We order the appropriate # of bill-only tests (as in Goodchild's x 27 example). We also record on the worksheet the spec # we billed on. If we were ever audited we could easily demonstrate why we were billing the 27 CPT codes.
It's also an easy mechanism to review the results and to catch the wastrel who does the 20 K typings to find the one unit, or types for c and E simultaneously and finds a half dozen E(-) units that we can't use because they're c(+).........
Besides telling the LIS, we put stickers on the units indicating the typing results, and that the typings have been billed out.