HQ staffs are 1.15 btw... which wont effect the relationship between MAB and agi. But yes like he said the more agi you have or the lower the mob int the less agi will be worth compared to MAB and vice versa.
Also that formula is a bit more off.
D = ((lvl+2+wsc)*fTP + 2*(agi-mobint))*staff bonus * day/weather bonus *MAB/MDB *whatever other bonus or mob weakness/shell etc
Where wsc is 60% and the lvl correct factor is .85 at lvl 85-90. Note you must round the values down each time. So agi *.6 round down *.85 then add with other stuff continue.
ftp=5.5
staff bonus, day/weather, MAB other bonuses etc work exactly like they do for nukes except of course you can't magic burst.
The 2*(agi-mobint) term actually has a cap but it's 1276 for the whole term so you will only reach it with brew.
As you can see this formula favors agi a bit more....
Easiest way to chose MAB over a agi peice would be just to plug it into the formula making slight assumptions on int (or you could back calculate it) as well as what your cruor buffs will be at the time. Though small differences between mob int and your agi will have a relatively small effect on the ratio of mab to agi for dmg so usually wont make a difference in gear choice unless it's something big or decent amounts of MAB.
Also if you start using it on nms could you report what you've been getting? Because every nm I've fought and used it on in abyssea and alot of these were low tier crap nms have taken anywhere from 30-70% dmg less than they should... using sekka and damn near one shotting the NQ for like 3.4k then hitting the NM for 1.1k really drove that point home.
Also fact that not everyone seems to know is that atma of the griffon claw does work with it and that's +20% ws dmg. And atma of the burning effigy refers to fire skillchain element not actual element...