My seed: -1015680097485353003
I wanted to know approximately how far I should be from land in order to have the farm work, because I have an ocean near my base, but all the slime chunks near me are still kinda close to land.
It's not how far the farm is from land that matters (unless you're using it as an XP farm as well), it's how far away you are.
So if you are 128 blocks away from shore then no mobs will spawn on land and you can have the farm just offshore as long as you stay within 128 blocks from all parts of the farm.
No mobs will spawn closer to you than 24 blocks.
No mobs will spawn more than 128 blocks away from a player and any mobs that get further away instantly despawn.
I don't know if under the ocean is still the best place for a slime farm now that we have drowned spawning in the oceans.