I would spend a fair amount of time reading the posts in these forums, and watching some youtube videos on bridge length, making bridges, splicing rope, sewing, etc. I would not put your butt 30' in the air hanging from stitches you just learned how to do without some real research. Please be careful.
The only advantage to me of a shorter bridge is less interference for shooting. Having said that, there's no shot I can't make with a 48" bridge, just requires a little maneuvering. Best thing to do is to tie one long, with climbing rope (figure 8 knot is really simple to tie). Hang from the tree at ground height, and grab both sides of the bridge with your hand right below the carabiner. Then slowly work your hand towards you, pinching the two sides of the bridge together. Once you feel it start to work your hips a little, stop there. Back up a few inches, and mark it. Then retie the bridge shorter, taking out the distance from your mark to the carabiner(x2). That should get you in the ball park. Most people will fiddle with this using knots and climbing rope as a bridge until they find the right combination of tether height and bridge length. Then if you want to tie a permanent bridge on, go to town.