So, I don't want to manually adjust my character's strength/intelligence scores when I use the order of the mutant mutagens that increase the score, and I want to make items that'll alter the scores for me, but I don't know how to get the increase to the score to scale with my level, I copied the snippet from one of them, but when I try to paste it into a homebrew item based on the manual of bodily health, it says its not a valid number, I've put the snippet below for reference.

(((characterlevel-4)/7)@rounddown+2)#max:5,min:3,signed"

Okay, three things:

1. Snippets don’t actually “do” anything, they just display calculated numbers. If the snippets could actually change your stats, then you wouldn’t need to create an item to do it.
2. Snippets don’t work in description fields, only work in snippet fields, and magic items don’t have those.
3. There is no way to make the items automatically scale like that. Magic items are not designed to work off of their bearers stats lake that.

You will have to add Modifiers to to the magic item to increase those stats, and then you will have to adjust the modifiers when your character increases in level.

Ok, that's fair enough, I was just hoping there was some clever trick I could use, I figured since things could scale off ability scores and proficiency, I could finagle it into scaling properly.

Nah, not with magic items., and not what you need it to do with Modifiers at all,