I made a set of tire warmers using that kind of wire. I too did some calculations but in the end, i got there by trial and error.
pick the amount of wire that you think is necessary. With aligator clips connected to your power supply, see to wich temperature the wire goes. If you want hotter, just move the aligator clips closer and vice-versa.
BTW, i didnt use denim, but coca-cola cans wich i cut using a dremel disk. Denim can get messy if you use tire aditive.
Good luck mate
