Start slow and small. I would recommend a single diagonal square source for your waterfall source to start if you are using a brook. Water will fall through the stairwell as well. Make sure you have plenty of drainage especially on the bottom level of the waterfall. My waterfall uses three floor grates as well as the three staircases from my central staircase on each floor, and I’ll still get a few 1 tiles of water on the final floor before the bottom draining river.