The differentiation you mention reminds me of the difference between Li and Jin in martial arts. I'll gloss them here, but I highly recommend the work of others to really understand the terms.
Li is brute force, like creating gigantic fireballs or moving heavy objects. It's character is 力, which fittingly looks quite a lot like a plow being drug through the earth. As you have noticed, this is easy to quantify and assign limits to.
Jin, 勁, is a bit more tricky. It captures the concept of skillful power. The character shows the idea of passes through, or under, like a river underneath a city. Me lifting a rock may be Li, but an Olympic grade deadlifter can only achieve those great lifts by power that goes through all limits of the human body that we thought possible, and empowers the lift with an energy that makes us all stand in awe.
And key to understanding how to balance these powers is the little I shaped symbol in the corner, Gong, 工. The Chinese meaning of this word is somewhat along the lines of "work." The first word of "kung fu" is this symbol (for phonetic reasons, sometimes it's written gong, other times it's written kung. But it's the same word). It's the concept of work and gains which comes from doing work.
Which brings to the test I recommend. Develop a sense not of how much power a particular spell has, but a sense of how much work has to be spent to develop a control over it. How many years did the villian spend practicing his skills at vacuum manipulation before the hero appeared. (Likewise, how many years did your dread pirate spend developing an immunity to iocane powder)
Finally, consider that anyone who develops this sort of skill through work is not going to waste it. If they have a particular power like this, they will want to use it for more than just a one-shot at the hero's head. They'll want to work it into their life. What does that look like? That tells you the shape of this gong fu. Don't focus on the brute force skill of pumping a vacuum. Concentrate on what flows through their life. Perhaps this means they focused on controlling winds around them. The vacuum is just an ultimate manifestation of winds pushing away from a point. How would they use that wind control in every day life? How would they not waste all that effort they spent learning it.
Then, use the rule of cool in the inverse. Is a character who acts this way "too cool" for the story? You'll get a sense of believability rather quickly.
And then you can also map this to how convenient circumstances are, and how little has to be done. Gandalf from Lord of the Rings is an excellent example of this. We get a sense of just how extraordinarily powerful he is, but he chooses to exhibit his skills only in the way which takes advantage of circumstances to decrease how much Jin, how much skilled power he must demonstrate.