Most professionals and vets now recommend fecal egg count. Have your vet do it (it's slightly more expensive) but you'll have the results and together you and your vet can come up with a deworming rotation that fits your horses shedding.
Don't over worm. You should deworm your horses every 3-5 months. I used to do mine every other month but after doing new research and talking with my vet, the parasites get immune to the wormer if you do them too often. Also, try to change the brand of wormer each time, this will also prevent the form of immunity. Hope this helps!