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?
Thanks!
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!
Best Answer
You can have slabs in the upper half of a block, or the lower half. Slimes (and other mobs) will spawn on top-half slabs. So, if you have three blocks of air, followed by a top-half slab, you end up with 3.5 blocks of air and a spawnable surface.
Also, of note - slimes spawn below level 40 in slime chunks AND in swamp biomes at any level, influenced by the phase of the moon. You can read more on the slime wiki page.