Silly question, but at what focal length are you trying to use it at?
EDIT:
I actually just picked up a 12mm myself, so for giggles I did a quick test. If you put it on a longer lens like 400mm, it doesn't change the focus all that much. I pointed at a wall which was 20' away on the top lens dial, and with the 12mm it shows it had to move to 35' to successfully focus.
On the short end, the difference is way more pronounced. At 100mm, 20' away with the extension tube the lens will not focus with AF or manual. So with the lens set at infinity, I moved it towards closer objects until something was in focus. With only the 12mm extension tube infinity focus is about 5' away. So the working distance I have is about 1' - 5' at 100mm, but at the long end it's about 2.5' - maybe 40'.
With a 25mm extension tube I suspect your working distances are even smaller than mine are. So the easiest way to figure out what they are is to set the focus ring to one end and move closer / farther away from something until you see it get in focus in your viewfinder. That will help you figure out what range you have. My suspicion is that you might only have like a 1' window at 70mm where it's going to focus.
I have the 5DSR camera and the 25mm tube however my 70-200 is the f/2.8 Mark II.
It does auto focus at the 70mm setting however the total range of focus is approximately 3/8 of an inch meaning that you have to have the camera distance very close to in-focus already before the AF is able to do anything. Its very finicky and seems impractical to use at least hand held. It may be of use in a tripod mounted macro focusing rail to hold things still and to adjust the camera distance more reliably.
For me I have never bothered to us AF at these high magnifications, preferring to only move the camera to manually focus thru the viewfinder, whereby the focus ring is only used to adjust the magnification which is not much with the 25mm tube at 70mm. The 12mm tube is more usable at the 70mm setting and switching to the 25mm for the longer focal length settings. I imagine that the 12mm tube does not do much at 200mm.
I routinely use a 12mm and/or 1 or 2 25mm tube(s), stacked or otherwise, with my EF135mm f/2, stopped way down, which yielded higher resolution with a softer background rendering in images compared to my EF100mm f2.8 USM. However, obviously, with way less magnification flexibility and the needing to switch out extension tubes.
I ultimately sold the 100 macro and still have the 135 with the tubes. YMMV (a lot).
