Starshards: which rank to use

Priest · Arcane · 7 ranks, 3 learned by level 30.

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

Most mana-efficient rank by gear: rank 3 up to 41, rank 1 from 42 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 10)	207 (207–207)	50	6 s	4.14	34.5	15 min
Rank 2 (level 18)	345 (345–345)	85	6 s	4.06	57.5	8 min 49 s
Rank 3 (level 26)	573 (573–573)	140	6 s	4.09	95.5	5 min 21 s

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

Rank 1: 207 damage for 50 mana in Forever instead of 112 for 50 in Classic Era (spell power share 1.002 instead of 0.624).
Rank 2: 345 damage for 85 mana in Forever instead of 204 for 85 in Classic Era (spell power share 1.002 instead of 0.924).
Rank 3: 573 damage for 140 mana in Forever instead of 333 for 140 in Classic Era (spell power share 1.002 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.