I have a small 10 gal compressor that has 2 gauges, one for the tank pressure and 1 for the output pressure. I just dial the output down to whatever pressure I want and hold it on the shock, the pressure will even out.
As said, you risk blowing your shocks without regulated air. With the tool made for the job there is no risk, can be adjusted on the road and can bleed off in 1lb increments, and you can check your pressure without losing any pressure. I played the air compressor game, had regulated it down to what psi I wanted, Did not seem to be the best option to me so I got the right tool for the job and I can take it with me.
putting in the desired pressure is easy. keeping it in is not. The shock has a low volume. as you remove the hose? from the valve stem some air leaks out. the pressure can drop a lot with a little loss of volume. The HD pump attachment puts air in but does not let any out. put an air gauge on the valve stem repeatedly and watch the drop in pressure.