For charged jumping, I think you should have the charge separate. It's important to be able to jump quickly at all times. Jumping when letting go of the key is the exact opposite of most games, it doesn't feel natural. Here's how I would do it:
1. Make the default height for the smallest jump a little tiny bit higher, so jumping without charging has more usefulness.
2. Make the character jump on pressing jump, not releasing it.
3. Have a separate charge control. I recommend ducking. When ducking, your charge meter rises. If you stand up or move left/right, your charge meter goes back down to zero. If you charge up and press Jump while you're ducking, you get the boost to your jump.
This sounds like a good alternative to have a quick jump button and a charging button. Ill see what I can do with it.