[ZeroGravitas]

This doesn't feel like a question asked in good faith but, 350 is a reasonable ppm target:

https://350.org/about/

edit: better link

https://mn350.org/understanding350/

[londons_explore]

I suggest 280 ppm is more akin to "pre industrial" levels, as that was the level about 300 years ago.

350 was the level around 1990, which is thoroughly "after we already messed it up".

[scythe]

I assume 350 is simply considered more achievable. It's not easy to remove carbon from the atmosphere, especially when you consider that a lot of it was not even in the biosphere before we pulled it out of rocks.