This is just flat wrong
For example, you can get an 80 tooth spur in either pitch, and would therefore run the same pinion on either gear, just different pitch

Gear ratio = spur/pinion
so if 64p is 100/40 lets say, = 2.5
the 48p spur you're trying to match is say, 90T 48P
90/2.5=36 - you run a 36 tooth pinion to get the same ratio
(40*.75=30 you would be 6 teeth off in this case)

All we mean by gear ratio is how many motor rotations it takes for one rotation of the spur gear, you have to spin a 40 tooth pinion 2.5 times to rotate a 100 tooth spur gear one time