Thanks for your replies. I don't mind filing down the my current bridge grooves carefully to get the action lower. I had read somewhere that people go with 10mm gap at the fingerboard edge or sometimes less for psychobilly slapping.. hmmm.... am I on the right track?
10 mil decent start - its not a science though. All has to do with feel, tone and songs. If you are playing slap mostly open notes then yes this is a good start - if you are slapping while playing tunes in a higher position you will have to raise that action up for sure. 3/8" is pretty universal i believe. Good luck. Use feel as your number one action guide!