A bottom leak is often a loose seal around the heating element from heat-cycle wear. Tighten the element and refresh the seal with a little silicone lubricant and it should stop.
We inflate lots of boards for friends and the routine never changes. Read the recommended PSI on the deck or near the valve, make sure the valve pin is up, set that number on the pump, and let it go but then the sound shifts when the pump moves from high flow to high pressure and that means you are closing in on the target, then either it shuts off by itself or you flick it off when the gauge hits the mark.
At the right pressure the deck feels very firm with just a tiny give under a hard thumb near the handle. If it feels like a brick you probably went too far, vent a little air and you are set.
