Devouring Plague: which rank to use

Priest · Shadow · 6 ranks, 2 learned by level 30.

At level 30 with 45 spell power, Devouring Plague rank 2 gets the most damage per mana and the most damage per second.

Most mana-efficient rank by gear: rank 2 up to 47, rank 1 from 48 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 20)	164 (164–164)	215	0 s	0.76	109.3	6 min 59 s
Rank 2 (level 28)	268 (268–268)	350	0 s	0.77	178.7	4 min 17 s

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

Rank 1: 164 damage for 215 mana in Forever instead of 175 for 215 in Classic Era (spell power share 0.800 instead of 0.504).
Rank 2: 268 damage for 350 mana in Forever instead of 295 for 350 in Classic Era (spell power share 0.800 instead of 0.504).

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.