I have used several different method, but the one I always come back to uses a strainght edge and shims to measure depth of dents.
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To Measure Dents
Set up a Grid
Place masking tape along the dent horizontally space each 75 mm apart
Do the same in the vertical with the tape 75 mm apart..
Measure
At the center of each tape intersection measure the distance between the straight edge and the can section using a taper gage.
Record
Write the depth at each intersection on the tape and record it on a chart like the one attached.
Chart/Analysis
Plot the data on a contour map or surface map scheme to determine maximum depth and shape of dent.
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I also use a General Tools Mfg Co. No. 837 Contour Gage