Wasn't it a percentage at some point in time...?
You have to experiment. No two people are going to have identical preferences. For example, I would not go higher than 23 for FHD (1080p) content - this is the point just before I start to be able to see detail loss, such as skin textures. To wit, I do my "fast" encodes at 22 and my NAS is loaded with encodes at 20.
For SD media, I drop the CRF by 2 (22 becomes 20, 20 becomes 18). But not everything is going to require that, either - the Japanese series of G1 Transformers, for example, I happily record at 24 because they're terribly mastered and just don't have that much detail to retain (I could probably go higher, but I'm happy with their size at 24).
A little note, though: the higher your CRF (lower quality), the more important it becomes to use better settings elsewhere. At CRF20 the difference between SubME 10 and SubME 7 is practically invisible to the eye and less than 1% for filesize, but once you get to CRF22 or above you'll start to notice it.
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