The precise dosage of Ivomec 1% for dogs is 0.0027 cc or ml per 10 pounds. This based on the concentration in Heartgard for dogs.

Heartworm preventive doses are so low that side effects are not produced even in Ivermectin sensitive individuals (some dog breeds such as collies)

a few relevant links:

http://www.beaglesunlimited.com/beaglehealth_canineheartworms.htm

http://forums2.gardenweb.com/forums/load/farmlife/msg0713514016840.html

Also please consult this very helpful book written by an experiences "Old Country Vet:

Old Country Vet    and  Heartworm Treatment

While dosing with the paste form of Ivermectin, which available in agricultural and feed store and online, is much less precise, it has been used in minute amounts without adverse effects.