I have looked far and wide for a comprehensive spur gear tutorial that uses module, number of teeth and pressure angle as the drivers, but I have had zero luck.

I have found examples that use workarounds or are cosmetic gears, but none that are parametric and true involute gears.

Some of the better tutorials (using diameter as a driver) get close, but fail to explain variables that require more information, such as one that said that dedendum=1.25*module. Where did that number come from? Similar questions arise around corner fillets.

Can anyone recommend a good source for this information? Or better yet, an explanation for the lack of information!