Okay, so as I was saying before, this is a simplified version of what I (and I think other mathematically-inclined people) do when they are trying to decide between INT or MAB in a particular situation. It's simplified in that, in order to make it less cumbersome for everyone, it makes a few assumptions.
I assume that a person is using T4 or AM2. The calculation for T3 would be different since the rate of damage increase as a function of INT is different. If someone really needs to know T3 and can't figure it out, I can adjust it and make another thing for that, too.
I assume that you have more INT than your target. This is reasonable, I think, because the handful of situations where you may not, I think other considerations will take over priority, like INT/Skill/Macc since those mobs tend to be more resistant.
I assume that you're just trying to decide between two pieces, one which has more INT, and the other which has more MAB.
This relies upon the fact that the relative damage increase from adding MAB depends only on the current MAB of the caster, and that while the relative damage increase from adding INT depends on more factors--the caster's INT, the target's INT, and the base damage constant of the spell in question--there is a reasonable range of possible base damages you can expect to see for any given spell.
So here goes: First, total up your MAB. To start, you get MAB from job trait (+32 for 75BLM, +24 for 75RDM and 37BLM, +20 for 37RDM, or from BLU job trait). Then, if you're a BLM, you get 2 for every potency merit in a particular element. Then add on the stuff from gear that says "Magic Atk. Bonus". Include the calculation here the MAB from the piece with less MAB but more INT that you're trying to compare, if it has any (like if you want to compare Yigit hands vs. Zenith for instance). Call this total MAB, "LOWMAB".
You can ignore the bonus from HQ staves, MB bonus gear, day/weather bonus, all that stuff. For this, you just want the total MAB for this comparison.
Now, repeat the same calculation above but with the MAB from the armor with higher MAB and lower INT. Remember to deduct off the piece you are replacing if both pieces have some MAB on them, like in the hands example above you'd subtract 2 from taking off the Yigit hands before adding 5 for the Zenith Mitts. Call this number, "HIGHMAB".
Finally, take the amount of INT on the High INT/Low MAB piece and subtract off the INT--if any--from the Low INT/High MAB piece, and call this number, "INTCHANGE".
Now, take the values you got for LOWMAB, HIGHMAB, and INTCHANGE, and replace them in the URL below:
Code:
http://www.wolframalpha.com/input/?i=(HIGHMAB%2B100)*x+%3D+(LOWMAB%2B100)*(x%2B(2*INTCHANGE))+solve+for+x
The website will give you a value for X. That value is the highest Base Damage your spell can have before MAB starts beating INT for damage. So refer to this chart below (dINT is how much more INT you have than your target):
Code:
dINT 0 dINT 50 dINT 100
Bliz4 506 606 706
Thun4 541 641 741
AM2 710 810 910
I made the chart up to dINT 100, but really that's not realistic for most things you'd care about, but you get the idea. So, in the easy cases, if you get a number less than 500, then the MAB always wins. If you get a number bigger than, let's say, 820, the INT always wins. If you get a number like 600, then the MAB is better for AM2, but you might have to try both pieces for Tier 4 to see which wins if you don't happen to know the target's exact INT.
I just realized this post is megahuge, sorry. TL;DR: Add up your MAB with the low MAB piece, call it LOWMAB. Add up your MAB with the high MAB piece, call it HIGHMAB. Take the difference in INT, call it INTCHANGE. Plug it all into the URL, and refer to the chart to see which piece is better. I'll do an example in a minute.