tensiometer and 2.16mm Ø spoke tension
Assuming you are right that their actual thickness is 2.1-2.2 and it's not a measurement error, that is an odd situation.
Assuming your chart has data for 1.5, 1.6, 1.7, 1.8, 2.0, and 2.3mm round spokes, the good simple thing to do is plot the curve of the reference numbers on the chart for a given tension in the relevant range, say 100 kgf or 1000N. Then find the appropriate spot on that curve where the average measured thickness is for your spokes. That's just a few clicks in any spreadsheet software.
This will give a lot more certainty than fudging it because the formula for deflection of round shafts has the smallest inscribed diameter of the shaft to the fourth power in it, hence the readout curve of tensiometers across different gauges is pretty darn non-linear. You could probably get to the same thing by reverse engineering the chart completely by using the formula to figure out what load the chart is figuring the tool is applying to the spokes, then making your own chart column for your spokes, but that's probably way more work than needed.