I mixed mine very weak and apply it often. I keep a spray bottle in my truck door and try to remember to mist my pants, waistline, cuffs and collar every time I leave the truck.
I put maybe an ounce of 2.5% into a quart jug and it has still proven highly effective at killing ticks. A little permethrin on your skin is not going to hurt you. I'll even mist my bare legs if I happen to be in shorts.