You're overcomplicating things; the ratings guidelines are in this area at least pretty solid on how to approach this:
— Pee is soaking part of someone's body or worn clothes? Questionable.
— No, but pee is visibly coming out or it looks sexualized? Suggestive.
— No? Safe, pee alone isn't anything.
Now, the comic does
make things hard because there's urine pretty much everywhere and a lot of puddling involved (which can go from Safe to Questionable pretty easily). I'll have to spend some time going over the images with tag mods.
I should also note "peegasm" stuff is likely to end up Suggestive, but that's dependent on a guidelines change, so not something to take into account right now