Never used boost with a 4.5t... only turbo.
5.5t on the track I run is good for full turbo (20), whilst the 4.5t I've had up to 14.. then caught a kurb and found a fence post!

I've run;
Motor: 4.5t
Software: 119MOD
FDR: 8.8
Turbo: 14
Delay: 0.3s
Ramp: 12/s
DDRS: 5

Thats it... honestly didn't feel it needed any boost at all, maybe that's just the motor.

HiH
Ed