Yeah what Pat said! I'm not too familiar with Finish a either, and only once roasted a sample up I got awhile ago, it was definitely interesting! Would love to hear more about the coffee you use
(origin, processing method etc)
As far as red line for extraction, I'm sure there is, but would probably need a refractometer to properly test it out if you've extracted too much etc and what time that occurs with the very specific variables you're dealing with (coffee, machine etc), hence why going with the visuals of the shot and taste is a good way! When it's gone beyond blonding point you can pretty well say that you're now extracting too much, and of stuff you don't necessarily want in your drink!
I think the generic guideline of roughly 23-32s or whatever it happens to be now is good, but that being said, I've pulled incredibly slow (50s) shots that were just divine, so it really truly depends! Go by what suits your taste and not what it 'should' be.
And also I have no experience with a lever machine so this may change things too.
Great question though