The magic attack formula is simple enough.
Magic Attack = 1.3(Int) + 0.2(Wis)
Concerning accuracy, after playing Mages of all modes and almost all levels (haven't done 59 or 60 yet

) I've found that Wis is not needed at all until you get to Map4 (Pando for AoL, Lanhaar for UoF). Your accuracy will be perfectly fine with just the natural bonuses you get on your armor or in accessories. 20-30 Wis? Perfect. Don't need any more.
Once you hit Map4, you'll find that your accuracy suddenly plummets. There's no mid-range. It goes from grinding in CL/FL and doing fine with no Wis to going to Pando/Lanhaar and sucking majorly.
Most Mages/Pagans start Pando/Lanhaar at about Lv48. By that level, you can farm Wis lapis quite easily, even Lv4s. But let's not even worry about Lv4s. Let's use Lv2s and 3s.
Wise Loop Lv4 x2 = 22 Wis.
Dread Set = 15 Wis.
Lv40 Pando/Lanhaar Weapon Int/Wis base = 19 Wis.
That's 56 Wis without even lapising. 96 if you have a healer.
Lv3 Wis x 4 = 28 Wis.
Lv2 Wis x 4 = 20 Wis.
Total Wis: 108 alone, 148 with Prayer Lv3.
That's definitely enough for introduction to Map4. As you get higher, you'll need a bit more, but as you get higher, your ability to farm Lv4 lapis will get even better.
Cost:
About 15 Lv3 Wis = 1.5mil
About 10 Lv2 Wis = 1mil
Set of Dread Gear = free for HM, about 12mil for NM
So for no more than 14.5mil, you've got all the Wis you need for Map4 and you can still go pure Int
And as I said, the further you get, the easier it gets to get better gear. Also, keep in mind that Wis is not needed in PvP. Real opponents don't have magical evasion and there are better options for gaining resist. On my Lv58 Mage, I have about 160 Wis when I wear my grinding accessories (about 140 with my PvP accessories). Once I hit Lv60, I'm taking all the Wis out of my gear and replacing it with HP, which is far more useful in PvP.