Yes, between 15 and 20 would be 20 feet. But before you waste a roll of film not knowing if the focus is accurate, why not calibrate your camera first?
If you have a piece of ground glass, take the back off the camera and lay the glass over the film rails on either side of the film chamber (w/ the ground glass side facing towards the front of the camera) and w/ the camera mounted on a tripod, or at least taped down to the top of something like a table, wall, something like that. The shutter needs to be held open for this. If you don't have ground glass, take some Scotch Magic tape and place a couple of pieces pulled tautly over those rails.
Then point the camera at something at least 100' away and get the thing focused clearly at that by peering at the image on the back of the glass/tape w/ a loupe. You can use any old camera lens turned backwards as a loupe. At that point you have infinity. Then to get the distances, use a tape measure and place something to focus on at the distances you're interested in, which might be 25', 15', and whatever the minimum close distance the lens can focus to. Make marks on the lens outer rim to know where these distances are, or loosen those tiny grub screws on the outer bezel and align the distance markings on them to correspond to the distances you have marked out. Those screws are often really tight and require a good screwdriver, so it's often a lot easier to just a marker pen to make your own markings.
That's about it. You have now calibrated the lens, and will get sharp photos, but remember to stop the lens down. Medium format lenses need this more than 35mm in order to give sharp pics. If your camera is relatively close to the markings as it is, just leave it be would be my advice, and stop it down a little more when shooting to cover focus accuracy.
