Timing gear ratio and number of female vs. male lobes for rotary screw compressors

[aegis4048]

I have an oil-flooded rotary screw compressor. This specific machine is female driven - motor shaft is connected to the female. This is one of the Leroi HG series.

Female has 6 lobes. Male has 4 lobes.

Assume gear ratio between female:male is 1. Assume female has 1000 RPM. This means female will have 1500 RPM due to lobe ratios.

Now I'm trying to understand the impact of timing gear ratios.

Questions, assuming female-driven 1000 RPM, and 2.50 gear ratio:

1) What is the gear ratio relative to? If 2.50 gear ratio, does it mean the male gear turns 2.5 times more than the female gear, or is it the opposite?
2) Assuming female:male = 6:4 lobe ratio, and female has 1000 RPM, gear ratio is 2.50, what is male rotor RPM?
3) What's the purpose of having timing gears when the RPM of male can be controlled by lobe ratios? I'm thinking that it's to make flow capacity & horsepower control easier should there be any changes - it's easier to just swap gears than changing the rotors. Please confirm this.

Thanks!