There are multiple factors that could affect the ideal mixture especially if you are conducting this on-site. Mixing on-site could pose more challenges, for example, you could experience more clogging especially with river sand. You will need to take into account your PSI and Rotar Stator which can differ your results. Also, note to be careful to not cut down too much on cementitious cement since that is not just your binder agent but your lubricant. We would recommend consulting with the manufacturer, Imer 301- 336-3700, since there is no easy answer.