If the customer does not want/need adjustments, a simple measurement from peg to peg will do for the peg slots on the strap.
If the customer wears his strap at various lengths, either the music, sitdown, standup, or switching guitars, you'll need to figure out an adjustment method, say a buckle, or a slotted adjustment strap, or other...your choice (and the bane of all strap makers). Many users want it all, without hardware to scratch their instruments.
Take a look thru the guitar strap section, for some different ways. I wish we could get the old pictures pre-crash, there were some good ones, if I remember right.