I usually shift two cogs smaller, unless my seated cadence is already very low. That's about 15% to 30% harder (21/27=1.28, or 28%) and I might drop my speed a little after I stand. I don't notice the cadence reading on the computer. This example, 70 rpm at the 27 cog, a shift to 21 would give 54 rpm (70/1.28=54.6)

I'm trying to balance over the crank, with little hand pressure on the bars, either pressing down or pulling up. It feels like climbing steps: I'm just using my weight on the pedal.

If I stand with the original cadence, I can't maintain it for very long. My heart rate would go way up.

I checked some of my Garmin recorded rides:

I have a 34-29 low gear. On an 18% grade, I can stay balanced and ride at 3 mph, that's about 33 rpm, or one pedal stroke a second. (60 rpm is two pedal strokes a second.)

On a less steep climb, when standing, I'll often drop into the mid-40 to mid-50 rpm range. That would kill my knees if I was sitting down.