Methods, systems, and apparatus for accurately determining a proportion (ratio) of two analytes is provided, as well as provide a concentration of a first analyte from a determined concentration of a second analyte and from a proportion of the analytes to each other. In one aspect, a surface model (called a “dose surface” herein) relating the concentrations of the two analytes to the proportion can be used to obtain accurate values for one of the variables (e.g. a concentration or the proportion) when the other two variables have previously been obtained. The dose surface can be a three-dimensional surface and be non-linear. The dose surface model can include multiple regression functions. For example, measured responses can be individually converted to concentrations using two dose-response curves, and the concentrations can be input to a dose surface function to obtain the proportion.