What size hole for compression elbow?

[RrogerD]

Going to be fitting a Titan electric shower on to tiled wall. It uses a 15mm compression elbow for the supply.
What size hole do I cut in the tile to permit the compression fitting?
Don't want to be too big as may be proud of the shower unit but also, if too small and not able to fit the fitting then I'm going to be stuck.
Thanks

 

[Exedon]

Most 15mm compression nuts are 24mm ish across flats bit more at widest
30 mm hole gives a small amount of clearance.