I was in a hurry to leave to work, so didn't read all the responses, so sorry if there's a already someone saying this: But You don't have to wait 1 sec after the 1st MP tick. Simply just take a look at your fire 3 cast time. is it under 3 sec? if No, don't spam the fire 3 button, just react to the mp tick, with human reaction time, it should give you enough time to get the 2nd mp tick. If your cast time is under 3 sec, calculate how much under it is. i.e if it's 2.70, you have to wait that 0.3sec after the first tick to get the 2nd mp tick, becuase from what I know, all MP ticks are 3 sec from each other. So, just go at a dummy, and practice the timing.
Also, nerfin your 1v1 dmg because your timing on aoe rotation is a little off? Ok, I bet your T6-9 runs lacks alot from that aoe rotation problem of yours.
And the "bug" where you end up with 1000 mana and not 1192, fire III costs MP when cast you know? (Haven't checked the mp cost during Umbral, so don't know if the mp cost is correct though, if that's what you meant)