Sundial drainage

[John Carmichael]

Hello all,   I have a commission to design a 5ft diameter horizontal mosaic dial for the island of Bonaire in the Caribbean.  Because of all the rain they get, the client wants me to incline the dial face slightly (but not a lot) for water drainage.   How many degrees should I incline the dial?  Is there some set standard that builders use for things like sidewalk drainage?   thanks   John