OK, so as I suspected, HP Recovery is added to your heal all the time. Good to keep in mind, since TOSBase didn’t have that adjustment. We start with 2145 average heal, roughly. After Resto 4, you get an average of 3160.
If we add doubled Additional Health Recovery to the formula, we expect 3313.3. That’s… more than expected. If the additional HP Recovery isn’t added at all, then you would expect 3160 – but that is cherry-picking our data. If we assume HP Recovery is added and INT portion doubled, that’s 3031, or a bit on the low side. Assuming HP Recovery is added in full a second time would be 3328, on the high side. Finally, there’s the possibility that magic attack is added, which would give you 3164 – but that wouldn’t seem to be possible given the one player who saw very little increase.
Odd that you also saw a large Resto level difference where Wurmheart did not. Think that might just be Divine Might being funky?
I guess I might end up making myself a guinea pig and doing Cleric C1 -> Krivis C2 -> Paladin C3 just to see how high heals will go without Cleric C2. Resto 15 and all. For science, and all that jazz.

