I just tried re-banding a gun with the Acid Green band from Neptonics following the formula for 380% stretch. I tried pulling the band back to the shark fin tab to see what it will be like in the water and I heard/ felt a tear between my sternum and rib before I made it. Not hurting right now but I expect some soreness tomorrow. Do you add a couple of inches to account for the width of the band slot and the waste beyond the constrictor knot? I assumed that would be made up in the wishbones. I'm hoping that pre-stretching will make this length usable but that made me feel like I need to turn my man card in for resignation.
looking at testing results from Neptonics the bands lose 6lbs of strength after 15 min of stretch but then it levels of more or less. I have read that some lose 15% upon initial stretch which gave me hope. Reading some reviews on another forum there were several claims that even with 380% stretch they seemed lighter tension, not my experience. I would recommend starting with 300-325% stretch then shortening as your strength determines. I have the strength to keep pulling but between the torn something or other in my rib/ sternum and knowing I'm repeatedly loading and unloading I'm going to salvage these bands for my rockfish gun. I considered halving them and tying in a second wishbone but at 4x stretch that gains me 8"/2=4" and I'm 12" shy. If I could make the tabs I would be able to shoot through a tree but I need to be able to dive and load this thing in the next week so I'll cut my losses.
A potential work around is to tie your wishbones a little longer to make up for the short band. then tie a normal length wishbone when the modified set up gets loose.
I have bands on my bluewater gun right now that are too tight because someone else tied them. I always go an inch-3 inches longer than what is suggested. It makes for a much smoother and more accurate shot, and you can always cut to make tighter.
I don't know if there is a way to "pre-stretch" them once they are cut like you asked. I hope so otherwise i have to throw away a set of perfectly good bands (except for being too short) on my gun. Let me know if you find out.
The instructions for those rubbers are pretty brief, sounds like you followed them OK. If your band slot is wide, it makes sense to add the width to the calculation. You need to be able to load all the bands in the water without too much trouble. I've had similar issues and will be trying a rest tab on the shaft or load assist to help get my hands back in a better position on the wishbone. On the Peace Boat we'll be loading and unloading each time we get off the boat (unlike baja). Looking forward to a great trip, see you in a week.