Its not that simple. How far under the floor all depends on the make and model of the tub filler mounting block you are installing. Although most filler mounts use similar mounting methods, the height of the mounting assembly dictates how far under the floor it needs to be in order for the top of the mounting flange to be flush with the top edge of the floor. For example;: This video is showing the installation of a Kohler K-T72790 which states: Secure the wood support / mounting platform between 2-1/4′′ (57mm) to 3-1/16" (78mm) below the top of the finished floor. By comparison, I'm currently installing a Landonbath tub filler and the instructions indicate the wood support / mounting platform should be at a depth of 2.3 inches - (58mm) kzfaq.info/get/bejne/ftCHadmKlrO8dGw.html

2x8

@@go-diy Thank You! Great video. Very helpful.

thank you so much!

As the go-diy indicated he used a 2x8 which is per Kohler instructions which state: "Adequate floor support is required. The wood support must be a minimum of 2′′ (51 mm) thick by 6′′ (152 mm) wide. 2x6 lumber is NOT adequate for this application." (this is because a 2x6 actually only measures 1-1/2" x 5-1/"2) hence the need for a 2x8.