I'll work on the BG wiki page tomorrow since I don't think the magic accuracy page has been updated in a while, but I could be wrong.
Below are the potential effects of magic accuracy and magic attack from the Mythic Aftermath.
Mythic Weapon Carnwenhan (Bard)
Magic Accuracy +30~49
Mythic Weapon Murgleis (Red Mage)
Magic Accuracy +30~49
Magic Attack +30~49
Mythic Weapon Tizona (Blue Mage)
Magic Accuracy +30~49
Magic Attack +30~49
Mythic Weapon Yagrush (White Mage)
Magic Accuracy +30~49
Mythic Weapon Laevateinn (Black mage)
Magic Accuracy +30~49
Magic Attack +30~49
Mythic Weapon Tupsimati (Scholar)
Magic Accuracy +30~49
Magic Attack +30~49
Ergon Weapon Idris (Geomancer)
Magic Accuracy +30~49
Magic Attack +30~49
Furthermore, the property “Weapon Skill+” and “Magic Accuracy Skill+” translates out as follows.
Weapon Skill +242: Attack +195, Accuracy +195
Weapon Skill +269: Attack +216, Accuracy +216
Magic Accuracy Skill +228: Magic Accuracy +115
Magic Accuracy Skill +242: Magic Accuracy +122
Magic Accuracy Skill +255: Magic Accuracy +128
Magic Accuracy Skill +269: Magic Accuracy +135
The above values should be used as a reference, as they are the stats that come out when applied to the base values. The actual value you receive may deviate slightly.
Taken from a response in the OF.
https://www.bluegartr.com/threads/112776-Dev-Tracker-Findings-Posts-(NO-DISCUSSION)?p=6713767&viewfull=1#post6713767