A method is presented to estimate the densities of He-like carbon and oxygen ions from the observation of the Li-like 3p to 3s transitions in the visible and the 3p to 2s transitions in the vacuum ultraviolet. The method is based on a zero-dimensional model for plasmas with short particle confinement times. General coefficients for this study are derived and evaluated numerically. The method is applied to Extrap-Tl reversed field pinch data.