Minn kota recommends you take the weight of the hull of the boat and add the weight of the max. allowed contents (motor, occupants, tackle, accessories, etc.)...devide this by 200 , then multiply by 5 to get the lbs thrust you need.

I checked this against my boat which is currently using a 36 lb. thrust TM and the numbers say almost 40 (which would actually be a bit better on windy days). So I think it's pretty close. Hope this helps.