Hi all! I am upgrading to a 75 gallon tank which is about 18 inches high. I have a wide hole cover so my uvb will be sitting on top of the cover. How far below can the basking decor be to where my beardie is getting the uvb he needs? Also, does my beardie sitting atop the rock count in the distance measurement because he is raised up 2+ inches or so than the actual decor pieces height. Thanks!!