According to this research video (skip to 4:30 ish)it's far more efficient to have 3.5m distance between layers in your slime farm. However, slimes cannot spawn on slabs, so how should I achieve 3.5m distance?
The idea is to pack a few more layers by using slabs and having 3.5m distance, instead of 4m distance.
And one more question:
At what level should the first platform be? Slimes spawn on layers below 40, does that mean the first platform should be at 39 or 40?
EDIT TO QUESTION #2
According to wiki
Slimes spawn in the Overworld in specific chunks below layer 40
Which should mean that layer 40 is not valid. I also set up a test platform at 40 and no spawns yet (sitting at 24 blocks from the edge of the platform). So I guess 39 is the answer!