The lines aren't always in the same place. especially in cages, practice fields, etc. Heck, we play on fields (mostly in practice) that have no lines. And the fields where there are lines are sometimes laid down by the park staff who drew the short straw and had to lay the lines at 6am Saturday morning.
I ask the girls to get in their stance, then hold the bat in their bottom hand and reach the bat to the front, outside corner of the plate. They should be able to tap/rest the bat on that corner comfortably (without stretching or bending), thus getting them the proper distance from the plate.