Please clarify gear reduction. Thanks.

I'm contemplating a DIY Ebike build using an EZip500 scooter for parts (24V 500Watt motor). I'm aiming for a low top speed, more torque setup for hill climbing. I'm figuring a 10-12 MPH max speed, does that sound about right for that purpose?

To reduce the RPM at the wheel, I'm going to use a jackshaft with a primary and secondary reduction. To calculate the RPM at the wheel do you __multiply__ the primary reduction and secondary reduction then divide the motor RPM by the total reduction? I've seen examples online where people were __adding__ the primary reduction and secondary reduction to get the total reduction then dividing motor RPM by total reduction. I'm a bit confused, can anyone clarify? Thanks, Mike