To address the point raised by Quietust, I ran the test at speedtest.net a number of times and found a similar relationship. My average up was slightly less than 10% of my average down (8.87%). I'm in the Netherlands, and noticed a similar relationship when I was in the UK, and, well, I think it's quite common.
I got the average observed down of 28.55 and average observed up of 2.53
Assuming this relationship of the up rate being 10% of the down rate we can say that the mean of the two values (up and down) is
(down + up)/2 = (down + 0.1*down)/2 = Down*1.1/2.
Or, calling the average asked for in the survey the Survey Value: Down Speed = Survey Value*2/1.1
So let's take one of my mean values (15.54) and apply this relationship to it:
Inferred Down = 15.54*2/1.1 = 28.25
Inferred Up = 2.83
(True values, Down = 28.53, Up = 2.55)
So, you know, it's not dreadful. And small changes (up = 0.08*down instead of 0.1*down) don't actually change the inferred down that much.
Hardly the best empirics, or theoretical mathematics, but might be useful for Skyrunner for finding a rough 'rule of thumb' number for Down (and for Up, albeit less reliable [due to decimal points]).
For a combined value, just a throwaway idea, how about Score = (Down - Average Down) + a(Up - Average Up). The Average up and Down here are the average for, say, your municipal district, or the average for the web forum you're comparing against, etc., basically the average values of your peer group whatever you choose it should be. The coefficient a is to account for the order of magnitude difference between Average Up and Average Down, say, if Average Up = 2.5 and Average Down = 25, then a = 2.5/25 = 10, in order to put both terms in the same order of magnitude.
So someone with comparatively a slow down, but a fast up speed might have a score of (10 - 25) + 10(5 - 2.5) = 10, which is a pretty meaningless number until we compare it against the scores of others.
We could also maybe use fractions! (10/25 + 5/2.5)/2 = 1.2 > 1. This number means more as a standalone value. 1 is on par, anything above 1 is good, anything below 1 is bad. This doesn't account for people's preferences though (I value Down far more than Up, for example).
Hm... I think I'm gonna make a thread in general discussion.