I read guides where if you hit 3043 haste you will benefit from extra ticks from a number of spells and if you hit another haste cap at 6652 haste rating, you'll get extra tick of your heals. Question is what is the extra throughput in terms of % do you get from that extra '1' tick. I know this also depends on the spell being used.
If you want to look at it as % gained from the extra tick, it is as you said, based on the spell in question and simple math.
At 3043 haste (and the raid haste buff), Rejuv goes from 4 ticks to 5 ticks, which is a 25% increase and the Tranq HOT (not the # of Tranq direct heals) also goes from 4 ticks to 5 ticks, a 25% increase on the HoT portion.
At 6652 haste (and the raid haste buff), Wild Growth and Swiftmend(aoe) go from 8 ticks to 9 ticks. That's a 12.5% increase.
That is of course, ignoring overheal - but you get the idea.