Just saw this thread. I've done it by both quantity and weight of lemons but have settled in on adjusting my Everclear to match my Zest or Peels. If you're lucky enough to have a tree or a neighbor with a tree then the amount of lemons isn't usually the problem, it's how much time you want to spend collecting the thin peels or zest. Once I've finished zesting I'll pour just enough EC to just cover the lemons. The extra EC's not going to go bad before the next batch needs to be made so save it. Funny, I was just talking with a friend from California last night and he mentioned needing some Everclear soon for lemons he still had hanging.
Mike
Mike