[kramerog]

I bought a stainless steel ruler on Amazon, then I took measurements filling my kettle up one quart at a time.  In Excel you can plot the points on a graph and generate the equation that most closely approximates a straight line.  Then you can build the function into a spreadsheet where the input is the measured height of liquid and the output is the volume.

This is what I do except with a painted aluminum ruler.  I feel that measuring from the bottom is more accurate than measuring from the top having done both ways as the end of the rule helps make the ruler stand straight when positioned on the bottom.

[Jimmy K]

I use an unfinished wood stick with markings. Wood gets wet and 'marks' the depth a little better than a metal one.

[dcbc]

I marked my wooden mash paddle with a wood burner/soldering iron.  One side for MLT strike volume, one side for HLT sparge water volume.  I didn't feel like adding an additional tool to locate on brew day.  98% of the time, i.e. normal gravity range beers, I don't change my strike/sparge volumes.  I just let my liquor:grist ratio vary.

Boil kettle volume for a 90 minute boil is the edge of the compression fitting on my whirlpool arm.

You can see the mark under the top hole in the paddle.