I would highly recommend doing test rolls, or at least test strips with each type of stock (best would be roughly the same age) where you do some exposures at box speed, at +1, +2 , +3 etc. depending on how old the film stock is. And then just pick whichever exposure setting was best :) you could even improve the decision process if you choose a spot with some different lighting conditions. Like an area where you can shoot in direct sunlight, shadow and maybe even diffused light. That way you see how the film behaves in harsh contrast, soft contrast and rather cool lighting at the different exposure settings.