A 1/32 less than 7/8 works for me. But it really depends on you fitting. Threads seems to vary by a 32nd or so. I like to keep it tight as possible. So I drill a little, then try it, then drill a touch more until I can get it where it almost threads in.
7/8" will work great.
I did 13/16" and the holes are so tight I couldn't put tape on the threads until the fitting was already through the hole.
No leaks though