Max RPM, Primary reduction, and 5th gear were pulled out of thin air.

Primary reduction is the gear reduction going from the crankshaft to the transmission cartridge. Then you have the Tranny gear reduction to the countershaft. Then the last reduction is the sprockets on the chain drive.

The primary reduction is a fixed value.
Tranny reduction varies with 1st-5th and Reverse.
The chain drive reduction is fixed unless you swap out sprockets.

Sometimes you'll hear the term "final drive ratio". That can be one of 2 things. It's usually the primary reduction & transmission gear reductions combined at each gear giving you the total reduction from the crankshaft through the countershaft.

Sometimes people include the chain drive reduction. But since that's an "adjustment point", I prefer to leave that out of the equation.