While there is going to be a correlation between the loss of friction material and the amount of fluid in the reservoir, there is not a direct relationship you can quantify. Every make/model/brand of vehicle is going to be a bit different as far as the size of the caliper piston and the size of the brake pad supporting the stopping power.
The amount of fluid is going to equate to the amount of friction material which is gone, multiplied by the surface area of the piston which is in the caliper. The size of the piston is what's going to relate to the amount of fluid missing from your master cylinder, not the amount of pad which has been consumed. The surface area of the pad does not come into play (the depth of pad used does, because the piston will travel this same basic amount).
Another factor to consider when looking at this is, most calipers used on passenger cars today have a single piston which is located on one side of the caliper. What happens when you have multiple pistons on one side of the caliper, or multiple pistons on both sides of the caliper. You could have pads with the exact same surface area, but could be utilizing far more brake fluid as the pistons move further out of their caliper bores.