Drilling $8 holes

Ok, let's say someone's working on a tub and forget to leave a gap where the shower valve shroud screws into the valve assembly (not that I would ever do that). In what I couldn't bring myself to call Friggin' Tub Demo VI, I found out the hard way. So, there are some mount holes somewhere behind tile, thinset and 1/2" of concrete wall board. First, I went to the hardware store and bought a $16 diamond tipped 5/16" hole saw designed for tile. Second, I took a wild guess where the holes should be and began drilling. I was only slightly off and, despite each hole costing about 1/2 of the $16, or $8/ea, that drill bit was worth every penny. Now I can mount the valve shroud in the next 5 months when I finally get to that point. Yay!