API GetSpellHitModifier

Returns the amount of Spell Hit %, not from Spell Hit Rating, that your character has.

hitModifier = GetSpellHitModifier

Returns

 * hitModifier : Number - hit modifier (e.g. 16 for 16%)

Example
/dump GetCombatRatingBonus(CR_HIT_SPELL) + GetSpellHitModifier;

and returns the Spell Hit Chance displayed in the paperdoll.