That's pretty much the case with whatever you install. You can add 1.5" by getting a round bowl, but your knees will be in the same location regardless.I will only have about 19 1/4" in front of the toilet to the finished wall?
If you have a wall heater on one side, you may want to move it out a ways from it. What you are mainly looking at is shoulder room. That's where your arms hang from.
You need at least an 11.5" rough from the finished wall for the Ultramax II.