Whilst everything that Flash has said is spot on, when adjusting the input pressure via an external reg, i prefer to wind the internal velocity adjuster (wether it be Rex or standard) into half way rather than all the way in as this does leave you with some slack if you set the velocity on a warm day and then play on a cold day. The cooler tank pressure could mean that you need more spring pressure to get the same velocity... Therefore, the ability to wind the spring in a bit can be useful.
If you have a WGP reg seriously consider the AA Vigilante or buying an external adjuster (try www.g3pb.com in the states (who DO ship wonderfully to the UK!))
If you have a WGP reg seriously consider the AA Vigilante or buying an external adjuster (try www.g3pb.com in the states (who DO ship wonderfully to the UK!))