For peak current, you need to know the inductance you're driving. The you use the formula of the integral of the voltage divided by the inductance. For motors, the peak current can by as much as 5 - 7 times this value when starting or changing speed under a load. That fact is going to make any snubber non-ideal, and so you have to make a compromise. Perhaps you want to use the"high" value of current for you calculations, as that will be the most critical conditions. I'm trying to learn snubber circuits too, and so I'm interested in your project. If you need help with the equations, I might be ale to help. Also, the site you linked does the calculations for you. You can just plug and chug.
Typically, the equations get you a starting point, and you need to do alot of experimentation to optimise the design. Good luck!