Re opening spigot part way.. You open it to just a drizzle and when you run water in the sink it will take longer to fill a glass than you care to wait, but when you close the faucet you may hit anywhere between 80 and 120 PSI ... if that is things do not go POP and flood your rig (Slowly) first.
What you describe is the bane of modern man, a FLOW RESTRICTOR, does notthing to limt pressure.
A pressure regulator has a valve in it, this valve is pressure controlled, when the pressure on the output side is too low the valve opens, the lower, the farther, to let water flow, when the pressure returns to the setting, the valve closes stopping (in theory) ALL water flow. Then when you open a faucet inside the MH, the valve re-opens, "Just far enough" to meet the demand wile maintaining the pressure.
Believe it or not,, Studying this is part of my Electronics training, you are looking at the difference between a voltage regulator (Varying voltage in, constantant (usually lower) voltage out, and a resistor. (Limits flow).
Home is where I park it!