I am trying to fit a typical ATPase activation or inhibition kinetic using e.g. verapamil or cyclosporine A. However, I have a hard time determining a proper EC50 (or Km) for the verapamil curve when I use conventional Hill fits (see attached file). The highly stimulated, bell-shaped saturation in my ATPase assay forces me to mask/leave out the last points. Is there a more elegant way to fit such curves or would it be enough to "stop" fitting at the max value and excluding the last data points?