We dont drill holes in our tubs. Our racks are home made and we have about 1/8" gap from top of tub to bottom of shelf above. I believe that is plenty for them. Dont want too many holes letting heat and humidity to escape.
You need to set up your tubs with the heat on, water bowl in, and the substrate you are going to use. Basically, set it up with everything but the snake. Now, measure the humidity inside the tubs and make holes until the humidity is correct. Remember it's way easier to add holes than to take them away, so take your time and get it right the first time.
Thomas "Slim" Whitman
Never Met A Ball Python I Didn't Like