Yeah I know how it works. I wouldn't be giving advice if I didn't.
If you NEED food to reach a minimum limit then you are undergeared anyway.
Printable View
If your gear is build around Det or Crit and just missing a bit of Acc to have the recomended you are not really undergear.
Easy way to figure out food: take your stat (after gear bonuses), multiply it by the percentage (which is the number divided by 100, so 5% is 0.05), and if the resulting number is higher than the cap, set it to the cap. That's how much you'll get from food.
The market boards are a good place to do research. The food on there is organized first by category (which stat it raises), then by level (lower level food has a higher % but lower cap, which means you should use low level stuff at low level, and vice-versa). Find the category you want, and find which one gives the most in the stat (Usually the last one you hit the cap on, or the one just after)
Similar to what Urth said, you can find out how much of a particular stat you need to get the full benefits of the food by dividing the max by the % (ie. 15(max)/.05(5% boost) = 300 of whatever stat to get maximum benefit). I rarely see people use food for speedruns, however, because the gains are usually marginal. The most dramatic difference a secondary stat can provide is capping accuracy, which should never be an issue in dungeons anyway. If you want some kind of boost, crit/ det food (probably more crit based for BRD) would be best, but I'd really only indulge in this if you are either making your own food or find something cheap (that is, not the same food you use for Coil), or if you just have no problem spending a decent sum of gil for running dungeons.
You shouldn't look at accuracy as a judgement stats for tank gears. It's secondary stats. I100 has more strength, more vitality and more defenses than i90 and that's what you should be looking at. Higher item level doesn't mean higher secondary stats since those stats might not be at cap.
...Any little boost can go a long way.
You remind me of this White Mage I had in Brayflox Hard mode...Who was in AF and all Lv30 intelligence accessories and belt. And then told me 20+ extra mind really doesn't make much of a difference. Meanwhile I was running out of MP as a bard keeping the WHM's MP up because their cures did absolutely nothing and we were unable to do any kind of "Speed run" tactics because he could hardly keep up with the tank having 2 mobs.