No, 60 is not half or a quarter etc of 90. You’ll get jittery motion.

If you’re running at 60fps then it would need to be either 60Hz or 120Hz for proper frame pacing.

You can cap in-game framerate to 45 and leave Steam cap at 90. The 45 divides nicely into 90 so you’ll get nice frame pacing. If the game doesn’t have a 45 option, then you can of course use Steam’s frame limiter. If you set the slider to 45, Steam OS will run the display at 90 but cap the game to 45, which also gives you smooth motion because of the even division. However, using the Steam frame rate cap usually incurs a latency hit, so prefer in-game framerate caps when possible.