It absolutely is not an exact science, Bernie ... It's all what you feel ... As chas suggested, play recordings of vocalists and listen to what it going on BEHIND them ... the right hand can take the place of a back up vocals group, horn fills, or just add some chord extensions ...
Just experiment and play ...

I occasionally perform with another vocalist or sax player, so I often practice just what I might play 'behind them' with my right hand ...