I use the factor method. 200% means that a 10 inch bands is drawn to a total length of 20 inches. Therefore, 100% means the relaxed condition.
The Thera chart is indeed using the elongation. Using that method, stretching 10 inches to 20 inches means 100%.
I find the Thera chart not very useful as they stop below the draw factors relevant for slingshots. Their target market is entirely different, so it is understandable.
For our purposes, the factor method is helpful as you only have to divide your draw length through it to end up with your band dimensions. 500% works well for a powerful TB gold setup (50-200 shots), so if you have a 30" draw, your active band length should be 6". If you want long band life, 400% is better, so 7,5" would be the length of the relaxed band (100-350 shots). At 300%, the shots will be much less powerful, but your bands will live 300-1500 shots.
The conversion is simple. Just add 100% to their values, or substract 100% from ours.
Jörg
The Thera chart is indeed using the elongation. Using that method, stretching 10 inches to 20 inches means 100%.
I find the Thera chart not very useful as they stop below the draw factors relevant for slingshots. Their target market is entirely different, so it is understandable.
For our purposes, the factor method is helpful as you only have to divide your draw length through it to end up with your band dimensions. 500% works well for a powerful TB gold setup (50-200 shots), so if you have a 30" draw, your active band length should be 6". If you want long band life, 400% is better, so 7,5" would be the length of the relaxed band (100-350 shots). At 300%, the shots will be much less powerful, but your bands will live 300-1500 shots.
The conversion is simple. Just add 100% to their values, or substract 100% from ours.
Jörg
