100iu /25 pounds sounds right to me.
Yes human vit E is what you want. Dog supplements tend to be low quality at best. Also look for a product that says "mixed tocopherolsand tocotrienols".
I give my 50 pound dogs one 400iu capsule every other day to average 200iu/day- as suggested by Mordy since liquid vit E in dropper is more expensive.