Drain Life: which rank to use

Warlock · Shadow · 6 ranks, 3 learned by level 30.

At level 30 with 45 spell power, Drain Life rank 1 gets the most damage per mana; rank 3 does the most damage per second.

Rank 1 stays the most mana-efficient from 0 to 300 spell power.

Every rank at level 30 with 45 spell power

Rank	Damage per cast	Mana	Cast	Damage per mana	DPS	Mana lasts
Rank 1 (level 14)	73 (73–73)	55	5 s	1.32	14.5	2 min 16 s
Rank 2 (level 22)	93 (93–93)	85	5 s	1.09	18.5	1 min 28 s
Rank 3 (level 30)	133 (133–133)	135	5 s	0.98	26.5	56 s

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

Rank 1: 73 damage for 55 mana in Forever instead of 68 for 55 in Classic Era (spell power share 0.500 instead of 0.390).
Rank 2: 93 damage for 85 mana in Forever instead of 108 for 85 in Classic Era (spell power share 0.500 instead of 0.500).
Rank 3: 133 damage for 135 mana in Forever instead of 168 for 135 in Classic Era (spell power share 0.500 instead of 0.500).

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.