It's a geographical thing. You need to talk to other beeks in your area about how much the bees typically need to get through the winter in your area. Here in northern Indiana most leave 2 deeps. One friend of mine leaves 2 deeps and 1 medium. In your area I would have no idea, but set a goal based on local wisdom and then feed them until the appropriate number of boxes are full and the bees won't take any more.
I don't think you can say well they need 2 deeps so that translates into 75 lbs of sugar, etc. because as they take your syrup, especially in August (around here anyway) there really isn't much of a flow going on, so they are going to be consuming a portion of the syrup and not storing it and that introduces an entirely different set of variables. . . what is the population, are they drawing comb. . . etc. etc. etc. So I don't think there's a formula whereby you can predict.
Dave