You don't. You learn how the games works such that you don't need some artificial rating to point to.
You know what your players and their characters can do. You in theory have been making encounters for them for weeks / months / years over all manner of campaigns. Just make a monster that falls into the same ranges and power as other monsters you throw at them at the level you wish the monster to be.
I can go into the Monster Manual right now... see the stats of a Hill Giant... and make a completely new monster whose stats are like the Hill Giant. Just change some numbers around, add in a couple "special powers", and then have my players fight it. Maybe it turns out to be an easy fight, maybe it turns out to be a difficult fight. But so what? Isn't that the point? Not all encounters are the same. So who cares what kind of "rating" it may or may not have? The PCs are fighting a monster, not a Challenge Rating.