Ratio of mass flowrates of fluid a and fluid b of differing densities is
Ka/Kb sqrt ((rho float-rho fluid a)*rho fluid a)/
((rho float-rho fluid b)*rho fluid a)
where rho represents density, and K value is a fudge factor known as the "flow parameter"
Perry's Chemical Engineer's handbook has background on how to this out, but I'd probably just ask the manufacturer of the rotameter for the figure.