I'll get straight to the question: does mining out the blocks above Y: 39 make their spawn rates greater (making the farm more efficient), or does it not matter?
For more context: a friend and I have been debating on what is the answer. He claims you must mine out the blocks above it for the greatest efficiency (I do know this is the case for passive mob farms), however in the entirety of playing this game I haven't once heard that, therefore I'm asking this question.
Up until 1.13, MC spawning checked from zero upto the top of the highest section that had a solid block so having nothing but transparent blocks above the spawn floor(s) increased efficiency. [One design (by gnembon?) kept the entire farm within the bottom section (0-15) for this reason.]
In 1.13, the spawning check is done on a per block basis so a block on 16 has a 1/16 chance to be used (vs the 1/32 chance in 1.12 down because 16 is in teh second section). Again, building lower is better, but the sections become comparatively unimportant.
1.14 spawning still seems to be being worked on (Hopefully, as it seems pretty well 'broke' at present…but is likely to be closer to 1.13 then 1.12).
Another consideration is how much the last few percent of hypothetical efficiency matter to one…
[gnembon & the rest of the technical players perform a valuable service by defining the uper limits of what is achievable, but these builds are more expensive (and often more productive) than many players want/need.]
As a case in point, I have a manual tree farm (dirt at y=63) centered above the spawn floors (top one being @ y=38 ) in the first area developed in my current world on the basis that keeping the farm loaded longer would more than compensate for the loss of efficiency.
Rollback Post to RevisionRollBack
WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.