First off, using it with a zoom lens is a recipe for insanity - the focus point will change as you zoom to adjust the framing.
Second, the longer the extension tube length you're using the closer you'll need to get to the subject to achieve focus (and the greater the magnification will be).
Start with the shortest (13 mm) tube on your 135 mm lens and see how much closer you'll get. Remember, by adding any tube to any lens you'll lose infinity focus. Then move to progressively more extension and see the differences. Just using the two longer tubes stacked with your 50 mm lens will get you to about life-size, and a 100 mm (or less) working distance, depending on the internal optics of the lens.