For the people who are or who are wanting to split the dose, isn't there a problem with heartworms in your areas? It wouldn't be at all safe to split it...
If Ellie were on Heartgard, she would be getting about 4 times more med than she needs because the lowest dose is for 25 pounds but any dog who takes Interceptor automatically gets 5 times more than what they need. Milbemycin Oxime is used in doses five times higher than it needs to be just so the drug also protects against intestinal parasites.
That is what I've read anyway. We do use Interceptor but it is all poisonous garbage.
I do not yet trust topical treatments for heartworms and Revolution and Advantage Multi fall into that category. They are both also pretty new which wouldn't be my first choice. Not tried and true...