Personal preference really, there is no single optimum number that will fit everyone. Different bows, different back walls, how hard/fast you pull into your back, etc., all influence where you set the release.
For example, if you're having problems with target panic -- in particular the variety where you have problems holding on the spot without wanting to jump on the trigger -- I'd suggest going even higher on the poundage setting to the point where it will be nearly impossible to get the release to fire. After a couple times trying to get it to go off you'll probably find you can once again hold on the middle without even thinking about it.
If no problems and just working on back tension, then you'll probably want to go lighter, but not so light to the point where you're ever at any time afraid the release is going to fire prematurely when you release the safety. If it does, then set heavier.
And don't keep yanking out the allen wrenches every time a shot feels like it takes too long, especially if you've had a few go off with what you feel is nice timing.....could be you not pulling through the shot correctly rather than a setting that is too long.
Depending on bows and what a person is trying to get out of the release in terms of working on their form, I know some who have their releases set just a few pounds over the holding weight and I know some who have their Evo's set up for double digits over their holding weight. Hence there really is no optimum single number that will work for everybody

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