Volumetric Extrusion settings

It's unclear to me after many Google searches exactly how to start with the built in volumetric extrusion of Marlin. I'm using Slic3r 1.2.9 and Marlin bugfix 1.1.x. Triffid Hunter's calibration guide seems to have written a blurb about using VE but it appears it was before Slic3r had support for it. If I understand correctly, by checking the box in Slic3r under

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Printer Settings->General->Volumetric E

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it means the g-code will be generated using extrusion values in mm³ rather than linear mm. In Marlin, the part that is unclear is what to send as the filament diameter to the M200 command. Do I use

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M200 D1.74

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for 1.74mm filament, or do I use the generic

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M200 D1.128

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(sqrt(1 / pi) * 2 = 1.128) as referred to in the RepRap G-Code documentation?