Lapping a plate perfectly flat is not critical. For a long time I used an un-lapped fairly flat ceramic tile. You could clearly see wear on the film where it was not making complete contact, yet I was able to get great edges.

I lapped the Ceramic tiles flat with a 325 DMT easily.

The glass plates also are easily lapped with a DMT and while flatness or lack of, may not greatly affect the honing edge performance, it will make the film more efficient buy using all of the grit on the film.

You can also increase film life by rotating your film or plate 180 degrees.