Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
I always just run clear base with a little black pigment
if you're using a mixing system, I find that 90% of the time (depending on fabrics) the pantone 'Cool Gray' mixes work great and match the intensity/level of the original pantone book color swatch. Makes for a nice, consistent way to get your grays.
What system are you using? We use CCI for both base and pigments. We have formulas for the full Cool Grey range. We use a D-Base/D-White mixture plus Blue 072 and Black pigment.
if you're using CCI's pigments, you can download their mixing software...