For ISE electrodes for monovalent ions at 25 °C , this value will be 59.16mV (some literature may show slightly different figures depending on temperature).

For ISE electrodes for divalent ions at 25 °C, this value will be 29.58.

Divide the measured value by the theoretical one and multiply by 100 % to obtain the slope % value.

The expected mV difference for two standards that are not a decade apart can also be calculated. The equation would be:

(Log(Standard Concetration 1) - Log(Standard Concentration 2)) x Theoretical Nernstian slope value

For example if you wanted to calculate the expected mV difference between a 0.5 ppm standard and a 1.0 ppm standard using an ISE for a monovalent ion, common standards to use with a Fluoride ISE which is monovalent, the calculation would be:

(Log(1)-Log(0.5))x59.16= (0-(-0.30)x59.16= 0.3x59.16= 17.8 mV

This information is located in this document: Introduction to Ion-Selective Measurement.