How to arrive at your number, practically.
Start from a chart. Several tyre manufacturers publish tables of pressure against rider weight and tyre width, and there are calculators that do the same. Put your real numbers in: actual weight, actual measured tyre width - and take the result as a starting point, not gospel.
Then adjust by feel, in steps. Drop five psi at a time over several rides. You are looking for the point where the ride is noticeably more comfortable and the tyre still feels supportive in corners. Go too far and you will feel the tyre squirming and rolling under you in a bend, and you will get pinch flats on impacts. Come back up a little from there.
Buy a gauge. Floor pump gauges are frequently inaccurate, and two pumps will disagree by a surprising margin. If you are chasing a pressure, you need to be able to measure it.
Measure the actual tyre width on your rims rather than trusting the label. A tyre marked 28 can sit at 30 or more on a wide modern rim, and that changes the correct pressure meaningfully.
And check before every ride or two - tyres lose pressure steadily, and the thinner the tube the faster.