Good advice on the Permethrin above.

Muskol is made in Canada. Canada has laws concerning the percentage of deet allowed. Some canadians I have met were interested in getting american insect repellent.

REI sells jungle juice which has about 98% deet. I'm not advocating using it frequently, as it can irritate fair skin, but it is strong. Does not completely stop the black flies either. You could get the pump and spray it around hat, collar etc. We put in a ziplock so less chance of leaking on other gear.

It's probably more of a mental attitude, but I don't know how you guys go in shorts and a tee shirt. I get nailed by the bugs also, but not nearly as bad as my wife does. We have gone in may and I don't always use repellent, my plan involves putting some clothes on. A hat and mosquito netting will help if you find yourself in a bad spot. A button down fishing shirt with a tee-shirt underneath seems to thwart most black flies for me.