This is the critical dimension for fit. $$S = \fracp2$$ Where $p$ (pitch) $= \pi \times m$.
You cannot measure tooth thickness directly with a caliper. You must use measuring wires (balls) or a spline gauge. din 5480 calculator
To "calculate" a DIN 5480 spline, you are usually determining the and the Inspection Dimensions (measurement over balls). This is the critical dimension for fit