I did something similar awhile back, but instead of going with a set amount of +Heal, I wanted to know what point of +heal I needed for haste to generate more heal throughput.
Our coefficients are slightly different. I'm also ignoring spell crit and downranking...
Things to note is that the HPS gains of spell haste are linear. The more +heal you have determines the amount of gain you get from spell haste, so the more +heal you have, the better spell haste is. For those of you just interested in the results...

* = Assuming to hit 3 targets
A negative number means spell haste is better with 0 +Heal!
I'm sure your wondering how I arrived at these numbers. To have 1% Spell Haste requires 15.76 rating. The equivalent amount of +Heal is 15.76*2.2 = 34.67.
Upon taking into account the HPS formula, +Heal coefficient, and spell haste you get this handy dandy formula, which I will plug in for triple stack lifebloom...
HPS of +Heal = (+Heal * Heal Coefficient) / Cast Time
HPS of +Heal = (34.67 * 2.025) / 1.5
HPS of +Heal = 46.8
+Heal Needed = ( (HPS of +Heal * Cast Time) - (.01 * Base Heal) ) / (.01 * coefficient)
+Heal Needed = ( (46.8 * 1.5) - (.01 * 900.9) ) / (.01 * 2.025)
+Heal Needed = 3,022.11
Here is the spreadsheet I used to work out these values. Our numbers seem to agree for the most part, remember that I am weighing in iLvl, so every point of spell haste is worth 2.2 points of healing.