I actually did this last Thursday in some free time, since I was expecting a busy weekend and Monday. You can't be in a rush for macro photos. The experiment was to see how much field of view the lens has at various magnifications. The easy way is to photograph a ruler. I suppose I should have done it horizontally, but it's tougher for me to adjust the vertical height of the tripod to keep the marks in focus. Easier to mount the ruler vertically, and tweak the rails as required so the mm marks are in focus. There's a reason they build 4 way macro rails.
All photos are 6000 by 4000 px out of the camera. The only Lightroom adjustments are to the lighting, there is no cropping or sharpening. The ruler is ever so slightly not vertical. At first the width of the ruler marks don't really matter, but as the magnification goes up, the width matters more an more. My counts are slightly rounded sometimes.
I set up the tripod and mounted the camera and 100 mm lens on the slider rails. I wanted to get as close as I could. We see almost 15 mm vertically, 22.5 horizontally. I should have measured how far the face of the lens was from ruler, but I didn't. Sigh. The specs page says the minimum focus distance is 30.5 cm, and I think that's measured to the sensor, and the lens is 12.3 cm long.
12 mm extension tube, a hair over 12 mm vertically, 18 horizontally.
20 mm extension tube, about 11.5 mm vertically, so not much difference.
36 mm extension tube about 9.5 mm vertically, about 14.25 horizontally. This one is ever so subtly not focused. I thought I'd nailed it, but I'm using live view, which is a small screen on the camera. If the shot is important, I'd do three shots, bracketing what you think is in focus with one slightly forward and slightly back.
68 mm extension tube (all three of them together, and I didn't bother with the various combinations) about 7.5 mm vertically, about 11.25 horizontally. This is a little under 2x mag.
The above shots are lit from a desk lamp from the side. The following shots are with the macro lens, using only the flash for lighting. It helped to have the desk lamp throwing some light while finding the focus, then turning it off for the shot.
1x, 13.25 mm x 19.9 mm. Compare to the first shot above, you can see the 100 mm lens by itself has a slightly larger field of view, and thus isn't quite 1x. Then again, the macro lens is a twist ring, and it's possible I was slightly over or under the exact 1x mark.
2x, about 7 mm x 10.5 mm, and thus roughly what I was getting with all the extension tubes together.
3x a hair under 5 mm x 7.5mm.
4x, about 4 mm x 6mm, maybe a little less. At this point I'm running the lights up against the desk drawers that I've got the ruler taped to, so it might not have been quite 4x.
5x, a hair under 3 mm x 4.5 mm. Tweaked the lighting slightly to get as far as the lens would turn.
Here's your reward, a macro photo of some fine hairs on the lamb's ears, or whatever they're called. Why this? Mainly because it's right beside the garden retaining wall so I could put the slider rails on it and be steady.
Here's the normal view of that leaf. It's about 2 cm across where I was shooting.