Turning the pressure switch on the top will just make the cut off switch off at a lower pressure. If you dont mind the pump running all the time (it will have a shorter life) then just TEE in a tube with a valve connected from the outlet of the pump back to your tank. You can then divert excess water & pressure.
this will just be a simple bypass valve, which will half the pressure
if you fitted a gate valve or it, you could adjust the water output to suit your needs (i.e 70 % etc etc)
a pic of the by-pass gate valve is on the DIY site, i cannot post a pic here as the pic has the URL on it, and its against the rules