I am to blame for the FRC formula, so maybe a little explanation is in place.
Calculation consists of two main components:
1) calculating the average air flow speed in the intake channel based on
- average intake channel cross section
- cylinder volume
- mean piston speed at given rpm, since the air flow has momentum and peak piston speed is not representative over the injection duration
2) calculating the fuel spray flow speed based on
- Bernoulli speed
- discharge coefficient
These two speeds are summed up to be able to get the time fuel takes to reach the combustion chamber while knowing the length of the intake channel (from the injector to the combustion chamber).