Controlling motor speed with PID how to scale PWM to RPM and back
No need to scale. You need to limit your PID controller output wit 0% and 99%, and find out what factors work best. There is I*R, all kinds of interference, acceleration, so voltage is not strictly related to speed. Actually sometimes it may seem like there is no relation at all. So you do PI or PID, and tune it. Best practice is to also use SI units everywhere, but not mandatory.

By the way, and this is very important. You must build motion profile. Which in case of velocity control means you have to gradually raise the velocity according to acceleration. Otherwise you will get overshoots and very unstable behavior, or poor control if you will lower your gain to calm system down.