Nobby has pretty much nailed what I was going to say. However I will put in my pennies worth.
I would not have a clue what %age a hill's steepness is.
I just look at it, run it and then decide what/how we are going to do the workout.
I will give you an example of what I did with a kid (16 years old) from late October to December prior to our National Schools Champs.
This student (Sam) cracked 3 ribs in early October (Mountain Biking .. and wore a tree !!).
He was pretty "Aerobically" fit at the time but of course once he went for a run breathing was a problem !. Luckily this improved immensely over the 7 -10 days after the bike accident.
Because we were then miles behind in our schedule, plus Sam had a heap of National exams just prior to nationals,I decided all we would do is effort runs over 3k , some light Aerobic Intervals for a few weeks and then we would do 4 weeks of Hills.
I "tweaked" the hills by running no further than around 200m,concentrating on ankle drive, springing from that with High knees.
We then walked down the hill after each one and then repeated.
We did fast stride work in a grassy strip nearby between the hills.
After 2 weeks or so of this I then had him running the 200 up the hill in around 35-36 secs, concentrating on that ankle drive.
5 days after having done a session of 10 X Hills Sam ran his first ever 800m in 2:03 on a grass track.
12 days later he ran 1:56.4 to finish 5th in the National schools 800m (After a 2:05 heat and a 1:59 semi)
At no stage did we go near a Track. Everything we did was on forest trails and roads.
I have to say I was slightly surprised as all we had been aiming at was getting Sam into the Final and we would look at next year(The age group he is in is Under 19) The fact he was in the mix at the finish was great for someone so young.
BUT : I have never forgotten the value of hill work and from that respect Sam's run was about right.
Also : Arthur told me in early 80's that it was best to take a kid into that meet 'undertrained', especially if they were not considered to be a contender.
Hope that gives you more of an idea.