With the smaller pellets it's common to feed them 6 pellets twice a day. This is what I do with Attison's Betta Pro and I've learned only from the best breeders on the forum :)
If you underfeed them you will notice. A couple of my fish from Thai breeders were VERY skinny, and after feeding them a substantial amount of Attison's, they all plumped up beautifully! You'll want to see their bellies swell right after feeding, but they should return to size in the next few hours. If not, then they are bloated, and that's a whole different problem lol.
Probably gave more info than needed, but it's usually better that way!
Edit: This is info for adult bettas only... not fry or juveniles, even though there are some similarities.