Sorry for the late reply....
When I shoot big reef fish with a medium sized gun (110 to 120 cm) I use three good quality 5/8 bands with a percentage elongation of 300% and a 5/16 shaft. If you can elongate the 3/4 to 300 percent elongation that will
work, if not use 3-9/16 bands. The fish in the image is a Dog Tooth Snapper aka....shaft benders.
Testing rubber speargun power bands Blue_Gun Here is a rubber test I did with a friend... not much has changed with the math but the Mean Green batch of rubber may not have been properly cured and would brake in time. Mori's rubber is now some of the best around.