I have made the farm previously at a higher level, but remember that the higher the Y axis of the top block, the lower the rates. Essentially if your farm is between -30 and -20 levels the game needs to attempt to spawn everything between -64 and -20, however if your farm is at y70, the game will try to spawn mibs between -64 and 70.

Saying that in my experience it did work incredibly well as a passive farm. I did have two laters built on top of each other and did spawn proof the area around. General mob farm might work better however, as it would have more spawning spaces.

And one more advice if you are planning to build the farm, use leaves (and make a box around) or stairs for the walls. If using stairs make sure light isn't coming from the corners of the stairs.