If you want the ultimate to rise exponentially according to the bossHP you could consider using a fraction of bossHP. like x/bossHP.
Here is a c3p if you want take a look at it. https://file.io/wanCf2zt1zJy
The function I used in the example looks like this in geogebra.
Im not sure if this is what you were aiming for but I hope it helps.