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.