Regrowth: which rank to use

Druid · Nature · 9 ranks, 4 learned by level 30.

At level 30 with 45 healing power, Regrowth rank 1 gets the most healing per mana; rank 4 does the most healing per second.

Most mana-efficient rank by gear: rank 4 up to 28, rank 1 from 29 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 12)	222 (215–230)	70	2 s	3.17	111.1	43 s
Rank 2 (level 18)	366 (354–378)	125	2 s	2.93	182.9	24 s
Rank 3 (level 24)	510 (493–526)	170	2 s	3.00	254.9	18 s
Rank 4 (level 30)	640 (621–660)	210	2 s	3.05	320.1	14 s

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

Rank 1: 222 healing for 70 mana in Forever instead of 223 for 120 in Classic Era (spell power share 0.783 instead of 0.550).
Rank 2: 366 healing for 125 mana in Forever instead of 396 for 205 in Classic Era (spell power share 0.783 instead of 0.726).
Rank 3: 510 healing for 170 mana in Forever instead of 567 for 280 in Classic Era (spell power share 0.783 instead of 0.783).
Rank 4: 640 healing for 210 mana in Forever instead of 717 for 350 in Classic Era (spell power share 0.783 instead of 0.783).

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.