The calculation to avoid involuntary drying out when anchoring on a falling tide is a lot easier than many imagine. Forget trying to interpolate between tiny soundings on the chart in order to guess the depth where you are. Just fill out the tidal height curve (or use the one on the chart plotter), see what the height of tide is now and note what it will be at low water. The difference is how much the tide will fall. It only remains to add this figure to the depth you want to be lying in at the bottom of the tide, sound in until you find it, and let go.