You are using the wrong button if its dropping multiple CMs with the switch in bypass. Dont use the "CHAFF/FLARE SWITCH" binding, use CMS left. Always worked for me.
As for optimal CM preset, against fox 2s I drop 5 flares with a very short interval, 1-2 bursts is usually enought to defeat any IR missile if you aerent in AB. If its tail espect just dispense a 3rd one. You simply dispense until you see the missile LOS rate shift aft as its tracking the flare. As for preflaring, I suppose something like 2 flares every 1-2 seconds would work, though I personally dont use it (as ive found it not to be quite as effective as just dropping your 5 flare sequence when you spot the smoke trail, its not hard.)