Bag on bar mounting question

im sure its a pretty easy formula, (Im not sure, so correct me if in wrong) figure out the weight of the rear of your truck. over the axle would be lifting the gross weight of the rear, then im guessing halfway down your bar would be double that, so on and so fourth, so if you had all the numbers, im sure you could figure out how much weight to inch (thats probally the wrong phrase) on the bars you'll be lifting. then you can figure out how much lift your gonna get too, but i prolly just confused ya, i think i just confused myself, haha so forget what i said and just find the forumla, lol