It really helps to know what's coming up to get the full value of your Foe. I pop it at the beginning of evey trash pack and end it when the majority of the mobs are below 25%. If I know a boss fight is coming up, i'll do maybe the first 30% of the trash packs HP. Or if the pack isnt too big, ill skip it all together. You always want 100% Mp going into a boss fight unless you have no casters and your healer isnt DPSing. I. Which case, you really dont need it at all. Perhaps do the Nin a solid for his elemental ninjitsu if you have one.