Can you post your spreadsheet please, as I'm a little surprised you've got Rejuv returning a 318 difference when it has a total coefficient of 78.4%, which means that you're adding 405 spellpower even if we ignore harmony.
Quick math time.
500 Int -> 606 Spellpower (Leather bonus, MotW, Spellpower raid buff. Ignored HotW because we can't assume people take it).
Account for coefficient
606 * 0.784 = 475 healing
Add in Harmony
475 * 1.25 = 594 healing.
500 mastery (assuming 20,000 Int = 24255 SP + 6206 * 1.1 from weapon = )
(4234*2) + 0.784 * 24255 = 8468 + 24368 = 32836
25% Harmony
32836 * 1.25 = 41045
26.04% Harmony (with +500 mastery)
32836 * 1.2604 = 41386
Difference: 286
Ratio Mastery/Int = 341/594
If we take the stat values into account: 682/594 = 1.15
Reversing that, for Int = 2 Mastery, Rejuv needs to heal for 28337.