Well, as always a good idea to ask your vet, because it would, obviously, be tragic for your pet to become heartworm positive, but, here is the study from Novartis(the makers of interceptor/sentinel). which does state the minimum does for heartworm prevention only to be: a "Minimum dose of 0.1 mg/kg"
and here's the link:
http://www.fda.gov/cvm/FOI/1365.htmIf you were to call up your veterinarian (which I would suggest), all it would take would be a tech to look up the dose in "Plumb's Veterinary Handbook."
PS if you do live in a year round HW hotspot you might also, if you reduce the dose of interceptor think about getting bi-yearly fecals because your area is bound to be a good area for intestinal worms as well.