I would argue that this is bad game design unless the random effects are all pretty close in power to one another, for precisely the reason you imply by this question: it’s impossible to set a level to.
Overpowered effects aren’t bad merely because they make the game too easy: they’re also bad because they are unexpected by the rest of the game’s design. Challenges for low-level characters usually assume that the characters cannot easily fly, teleport, or dig through solid material. They also assume that they won’t have any defenses against or remedies for things like, e.g., petrification, outright [Death] effects, or imprisonment.
So a spell that has a 1% chance of petrifying someone at low levels is bad not because that’s a very powerful effect: it’s bad because low-level characters cannot easily get stone to flesh. You have a 1% chance of breaking the game with a spell like that. You cannot balance that out.
But if you put the spell’s level high enough that even the worst case scenario isn’t too bad, you now have a spell that’s usually far too weak for its level.
And this ties into the general idea of random chance and swinginess. A game uses randomness to keep things interesting, but ultimately the amounts of randomness has to be very carefully monitored. A player being very clever and finding a way to stop the BBEG’s evil plot several levels early is bad, but at least rewarding for the player. But if it’s sheer random luck? Not particularly engaging for the player, and really boring for his teammates. The flip-side is also true: dying is never fun, but if you were fighting it out, and maybe you made took a risk when your HP was getting a bit low, and you got killed, or you died fighting for something too important to run, that’s at least a death you can accept. It’s something you can prepare for, try to mitigate, and that you can take actions to try to prevent. A random, 1% chance of your character just being wiped out, though, is not something you can do anything about. It’s just an arbitrary “sorry, you get to sit out for the next couple of scenes.”
None of this strikes me as a good thing to add to the game.
So try to keep the effects relatively close to one another in power, so that if the worst-case happens, well maybe it was a little early for that effect but with some effort you can solve the problem.
And you’re going to have to put the spell’s full stats here for us to really be able to even try to give it a level.