I have 3 x 1550mah 75c 3S1P 17.2Wh, what would be the ideal voltage level I should set my beeper to? 3.4v per battery? Higher or Lower?
3.3v - 3.4v / cell (not battery) is good - so that's 9.9v - 10.1v for the battery, as that gives you a little time to get back and land.
3v / cell should be regarded as the absolute limit.
I agree with Normoa. I have my beeper go off at 3.5v when it goes off during a hover, that's when I bring it in. Rests about 3.6-3.7v which is good in my eyes. Remember sag will make the beeper go off during punch outs and high throttle moves due to sag.