Heal: which rank to use

Priest · Holy · 4 ranks, 3 learned by level 30.

At level 30 with 45 healing power, Heal rank 2 gets the most healing per mana; rank 3 does the most healing per second.

Most mana-efficient rank by gear: rank 3 up to 35, rank 2 from 36 to 73, rank 1 from 74 healing power.

Every rank at level 30 with 45 healing power

Rank	Healing per cast	Mana	Cast	Healing per mana	HPS	Mana lasts
Rank 1 (level 16)	342 (320–363)	155	3 s	2.20	113.9	29 s
Rank 2 (level 22)	460 (431–488)	205	3 s	2.24	153.2	22 s
Rank 3 (level 28)	570 (536–603)	255	3 s	2.23	189.9	18 s

HPS counts the global cooldown. Mana lasts: casting only that rank from 1,500 mana.

Rank 1: 342 healing for 155 mana in Forever instead of 363 for 155 in Classic Era (spell power share 0.857 instead of 0.729).
Rank 2: 460 healing for 205 mana in Forever instead of 515 for 205 in Classic Era (spell power share 0.857 instead of 0.857).
Rank 3: 570 healing for 255 mana in Forever instead of 651 for 255 in Classic Era (spell power share 0.857 instead of 0.857).

From the client, for each rank: base damage or healing and its range (SpellEffect base points and variance), the extra per level up to the rank's max level, the spell power share (coefficient), mana cost, cast time, duration, global cooldown and cooldown.
No downrank penalty in the Forever client. Classic Era's client puts a penalty for spells learned before level 20 inside the coefficient (Shadow Bolt rank 1: 0.14); Forever's client gives the full share (0.486), for 80 low-level ranks across the classes. Anything else would be on Blizzard's servers, so the calculator shows Classic's rules only as Speculation.
Speculation: crits count only on direct hits, at 150%, as in Classic; the example gear (1.5 spell power per level) is not client data.