A lot depends wether your car is on a standard computer or not.

For a standard RST a -18 actuator is fine (standard Cosworth actuator).
It is using a stronger spring then the std Escort item and will hold boost better.

The ignition for my S1 is cut off at about 8 PSI, so a -31 actuator is of no use.
I would end in the cut off all the time when trying to get the boost higher.

If you have the later RST ECU, it allows higher boost,
somewhere between 10-14 PSI.

Then a -31 atuator could be an option, although a std Cos item would do the job.
Please check wether your fuelling matches the increased boost.

About the setup: this really is a trial and error matter.
When the ignition is cut off, you went too far.

The standard test for turbo pressure (from the workshop manual!):
drive in 4th at 1500 rpm, floor the throttle until you reach 3500 rpm,
keep full throttle and brake hard with your left foot, read the boost gauge.
Max time do do this is 5 secs.
An open en deserted road might come in handy....

All this applies to a standard computer, modified items need tuners to set them up!

Cheers