The glomerular filtration rate (GFR), the maximum rate for tubular reabsorption (Tm), and the plasma concentration (P) are known values for a reabsorbed substance. The amount of the substance appearing in the urine per unit time, at a plasma concentration above that for saturation of reabsorption, would be:
P (GFR) - Tm Tm (GFR) - P Tm (GFR) + P Tm - P (GFR)