Shadow Word: Pain: which rank to use

Priest · Shadow · 8 ranks, 4 learned by level 30.

At level 30 with 45 spell power, Shadow Word: Pain rank 1 gets the most damage per mana; rank 4 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	Damage per mana	DPS	Mana lasts
Rank 1 (level 4)	84 (84–84)	25	3.36	56	1 min 30 s
Rank 2 (level 10)	114 (114–114)	50	2.28	76	45 s
Rank 3 (level 18)	162 (162–162)	95	1.71	108	24 s
Rank 4 (level 26)	234 (234–234)	155	1.51	156	15 s

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

Rank 1: 84 damage for 25 mana in Forever instead of 48 for 25 in Classic Era (spell power share 1.200 instead of 0.402).
Rank 2: 114 damage for 50 mana in Forever instead of 94 for 50 in Classic Era (spell power share 1.200 instead of 0.624).
Rank 3: 162 damage for 95 mana in Forever instead of 174 for 95 in Classic Era (spell power share 1.200 instead of 0.924).
Rank 4: 234 damage for 155 mana in Forever instead of 279 for 155 in Classic Era (spell power share 1.200 instead of 1.002).

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.