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one thing that’s a bit confusing is it was emphasised that I needed to measure From the muzzle to the furthest notch from the handle I.e not the notch I’m actually firing from. Any ideas why this may be?
Probably because:
1. Most people load to the furthest notch for more power (you seemed blessed with flatties - for which lower power makes sense. Btw which county are you diving in?)
2. The formulas concernd max. stretch and the furthest notch will cause most stretch. You - or somebody else - may later opt to load to the furthest notch for a bit more power.
If you will never load to the back notch then, yes, measuring to the notch you actually use is a viable alternative. That would give you a bit more power and resistance but if you want that, why not just load to the back notch?