One of my earliest posts was on super heroes when I introduced my Rule of 15. The rule was my attempt to codify powers and abilities beyond those of normal humans for my games and establish a power level I felt comfortable running in a game.
The Rule of 15 says that an amplified ability is fifteen times as powerful as a normal human's ability. An exceptionally fir man might lift 200 hundred pounds. An exceptionally fit man gifted with super strength would lift 3000 lbs. about the weight of a very large car ca. 1938. Check the cover of Action Comics #1 if you don't know what I'm talking about.
Take running. A normal human might max out at 25 kph. A super running power would let you run at 375 kph (about 104 meters per second). That's fast enough to run on water or jump a 10 meter wall (104m/s divided by 10 for the time it takes gravity to notice you are airborne).
The rule works well for a World War Two supers setting. It gives a decent power level while preventing your caped draftees from flying to Berlin and trashing der Fuehrer. Although it might be argued that would help the Nazi war efforts if done at the correct time.
How to model it in an old school system though? I'm thinking in particular of The Front and other WW2 rule sets to follow.
First there should be levels of power. Not all people with super strength are equal. Spider-Man should not arm wrestle Thor, for example.
Second there should be some chance for normal humans to prevail against super humans through training numbers or sheer luck. Otherwise what's the point of minions?
A quick look at the D&D Wiki tell me that every 10 points increase in strength quadruples the character's carrying capacity. I can also see that the Golden Age Superman without his powers has a strength of 15 which seems to fit his appearance back then. I could set my power levels to +10 and +20 to the characteristics easy enough. But when do I do things easy?
Let's start with Super Strength. There are three levels.
Low: (still within human levels but looks unlikely. Think Conan.) The character's fists and feet do 1d6 in melee damage.
Paranormal: (not within human limits, think Conan on PCP or the black lotus if anachronisms bother you). The character subtracts 5 from any strength related rolls. His fists and feet do 1d8 in melee damage. the character's blows may affect an unarmored vehicle.
Inhuman: (think someone who could bitch slap Conan on PCP). The character subtracts 10 from his strength related rolls. His fists and feet do 1d10 in melee damage. The character's blows may affect an armored vehicle.
In addition to the bonus to strength related rolls and melee damage characters with super strength roll an advantage die in any rolls made against characters with a lower level of super strength (or no super strength).
Next: Super Dexterity.