Pitch itself does not affect gear ratio - the number of teeth (or more precisely: their ratio) is all that matters. 100/25 is the same as 80/20 or 60/15, regardless of pitch.

The "conversion" is only needed if you want to keep the same size gears (in diameter, not teeth) in a different pitch. And there the formula /1,5*2 is just a shortened form of /48*64 - since gears with the same number of teeth, but in a different pitch, will be different in diameter.