There's quite a few abilities that require the enemy to have a certain percentage of their max hp to work. Currently there's a bit of guesswork on that, and it can make a huge difference - Rotos at 51% will wipe the team out, vs Rotos at 49% is toast.

Just add a simple thing to the info screen that shows exactly what percent of max hp has been reduced, and what percent is remaining. The rest can be solved for with pretty easy math.