We do this all the time and Sonny's suggestion is what we do 9 times out of 10. Bonus is that you can use the same screen for anything getting a standard location in the run.
On your film template, put a simple vector of a shirt in there and you can even give positioning instructions right on the work order.
On press, lasers are awesome for this I just stuck a couple harbor freight ones on the bottom of our gauntlet's control box for a run today and it was super helpful. Fat sharpie lines on the platen are fine too. The key is to use good reference points and also make sure your client knows how the print will look different on a small v. a 3XL with this print location. Make sure you or you staff slows down at first until they have the hang of loading the garments that way to prevent crooked prints and prints on the wrong side which happens even to me more than I care to admit.
