"HP maximum reduction" is almost entirely fluff as far as CR is concerned; what matters is the damage. Rounding error.
+12 damage per round sustained over at least 3 rounds, +36 damage one round (both before factoring in accuracy), 32 HP, +4 AC, +4 to hit/save DCs are all worth about 1 CR.
If you are replacing an other ability, it is the change that matters. So if the monster's best option was a multiattack for +11 to hit/3d6+10 damage, that is 41 damage per round. If you add in a new ability that does less than 41 damage, it is a wash. If you add in a 1/encounter ability that does 41+36 = 77 damage average, that is +1 CR.
If that 77 damage ability is a medium sized AOE and save for half, you'd double it for the AOE (to 154) then throw another +50% at it for save for half (231), subtract the 41 (190) then divide by 36 (5); that ability is worth about +5 CR, give or take.
It gets harder when you throw in stuff like status conditions, but for basic damage it isn't hard to adapt CR.
(be careful of HP multipliers from immunites, however. A creature that is resistant to everything gets 1 CR for every 16 HP, not 32, as their HP are effectively doubled.)