The elastic you find on most lingerie (like garter belt straps and bra straps) is typically a plush-back picot elastic. It is only intended to stretch about 40-50%. This is why most bra and garter straps also have an adjustable slide. If those straps were more stretchy then they could no longer provide the intended support of holding things up. You need just enough stretch to compensate for movement and comfort.
Braided elastic (middle of the pic) is typically what you'll find at the craft store and use in your costumes. It's far more stretchy; it's intended for waistbands and other such elasticized channels. It can stretch nearly twice it's length and is not suitable for bra or garter straps.
However, attaching a ribbon (with a short, straight stitch) will greatly reduce the stretch of a braided elastic which, then, makes a suitable garter strap. The shorter the stitch, the more the stretch, but it will still be slightly less stretch than the lingerie elastic. Of course, the ribbon prevents attaching any sort of adjustable slider.
But that's okay! Because the floorshow garter belts I sell are made from spandex. I cut them out so that they stretch more vertically than horizontally. This means that the garter belt itself is compensating for the rest of the stretch that the ribbon-stitched-on-elastic has lost.
That all being said, there's no perfect strap length that will fit everyone. Some people prefer to wear their hose higher than others, different torso lengths, etc. Best I can do without fitting it on you to see where the garter belt will 'ride' is aim for an average length. The wearer can then adjust where the garter belt sits around the waist/high hips, and adjust the height of their stockings accordingly. My straps are shorter in front and longer in back (to accommodate the way the body moves).
