Milling T-track slots - sanity check please?

[WillAdams (William Adams (Carbide 3D))]

Dimensions for what Carbide 3D used were drawn up at:

T-track extrusion

Had occasion to draw this up: [t-track-extrusion] I’ve found that 1/4"-20 hardware (Imperial) and square M6 (metric) fits well, albeit a little loose — anyone know the nomenclature for the appropriate size(s) of T-nuts? (I’m guessing it’s different for Imperial and metric) — are there clamp kits which fit especially well?

(the ones I got from Woodcraft were similar — I believe the ones you have are billed as “low-profile”.