I would always go for 3 sand to 1 cement mix , mixed semi dry ( just wet enough to form a snowball in your hands ) the easiest way I have found to mix my screed is to soak the sand first before mixing . try not to add water to the mix at all . and do not leave the stuff in the mixer to long or it will turn to balls ( literally) Alternatively get the customer to pay for ready mixed screed . the stuff will be a joy to use and all you have to do is get it into the floor. PS do not forget to pre-treat your concrete sub base with ideally a PVA/cement slurry before laying
Good luck with the job hope you don,t get sore knees