like FAR, but not too much because it will not load(cause of my vision)
You want to be around 20 to 24 blocks from the spawning platforms in order to still get spawns and allow them to walk around so they can fall down as necessary. Anything more than that and they may spawn but won't move around and depending on how far, may not even spawn.
Vanilla mob spawn mechanics, all spawn locations within 128 blocks have equal chance for the spawn to occur. For your spawner to be "Effective" you need to light up all surface in that radius and caves…
That being said… When I did my spawner early on, I realized shortly after its completion, that almost all the drops I wanted I was getting with seeds anyhow…