The interface is good. If you enter the distance and weight, and increase age, the assessment gets better and better, which at least intuitively makes sense. The weight is used on the VO2 Max calculation, but not in the fitness assessment, which I think you could argue about either way.
I didn't do a 12 minute run -- I did a mile and it took me longer than 12 minutes to do it. This calculator thinks that was very poor (which I'm inclined to agree with), with this proviso:
"The validity of this test is quite good if you are accustomed to running. For people not used to running it can be difficult to administer the speed from start to end and also peripheral discomfort in the legs can limit performance rather than circualatory (sic) capacity."
My excuse, of course, is the lower back stabilizer issues (which used to be sciatica when I was pregnant). Matters are slowly improving; I'm optimistic I might be able to run a mile again some day. Altho if I never do, I won't feel that bad. I never used to run anyway because of my respiratory problems; I just walked fast uphill instead.