I find a single dose of 300mg in the morning waking up with withdrawals from the heroin will rid me of all symptoms and give me a pleasant mood for the day, if I went particularly heavy (I ordered some uncut (well 68% pure) heroin and that left me ruined with withdrawals) I will need a top up of say 150mg around 9pm so i can sleep, then wake up and have another 300mg, the second day I only need one dose and by the third morning all the heroin has left my system and i only need 150mg to be well for the day, by morning 4 i don't need anything, I am technically cured.
as for your kratom dilemma,
I would advise heavily against a long taper, with the dhc i would aim for 4/5 days max
I guarantee you most of the withdrawal is self fulfilling, you are expecting to feel bad for that long, you wont, but if you think it you might make yourself feel shit.
and your dosing regime is off man, dont dose 90mg 3 times a day, dose 300mg in the morning and see how you get on, i guarantee you wont need more, if you do then take 60mg before bed, you will prob wake up feeling ropey so do the 300mg again, try train yourself to only use one dose, its mostly a mental thing, trust me,
at least give it a go and if it doesn't work for you then try your method but I honestly think a short sharp taper is the best way to go, whats the halflife of mitragynine?