The "Ideal" number is just what has been agreed upon by people to be a good final drive for that class and motor, final drive doesn't have anything to do with transmission/spur/pinion, it's all based on how much torque vs. speed is generally wanted for that class(taking into account tire size), and what the specified motor can do for torque/rpm. Final drive is the ratio of rpm seen at the motor compared to that seen at the wheels, so 8:1 is 1/8th the motor RPM at the axles/wheels.
(Spur) (Transmission ratio)
Final drive = _______________________
Most trucks and buggies have a 2.6:1 transmission ratio(Associated/Kyosho/HPI) and Losi uses 2.43.
But the charts aren't always right, it says in my T4 I should have had an 18t pinion, and it says the 16 I ran was 13% under geared, but it got hot enough after every race as it was, but never "hot", I believe the 18 would have been pushing it a bit. Anyways, thought I'd try to clear up where they get the ideal final drives from, it's nothing completely technical, mainly just what they've found tends to work good for a specific motor in a certain class.