If you thin the paint a touch, you shouldn't notice any appreciable loss of detail Michael. To make the dark grey, you are better to start off with basic colour, that is, black and white, rather than starting with a grey. This is because some grey paints have a blue cast, and others a green cast, which can become visible when mixed.
I used Humbrol enamels, adding just a few spots of matt white to matt black. As I did it 'by eye', I have absolutely no idea of the exact ratio, but it would be something in the region of 6 to 8 parts black, to 1 part white, probably less on the white.
You might have noticed, in the Halifax thread, that once the gloss clear coat was applied, the dark grey appeared black (the normal effect of applying a gloss clear coat to any mid to dark-toned colour. Once a matt clear coat is applied, the matting agent 'softens' the effect of the gloss, returning the colour to very near the original).
This can be a useful way, although not foolproof, of 'testing' your mix. Paint a piece of scrap plastic in the desired mix and, when dry, wet it - a wet finger tip, wiped over the painted area will suffice. If it looks black, then it's acceptable - if it still looks grey, then too much white has been added to the mix, and can easily be modified by the addition of more black.
Trial and error is the name of the game, only adding small quantities of white at a time, until the desired result is achieved. Once you've got it as you require it, this experience will aid you the next time you do it, and so on, until this experience in mixing will make things quick and easy to do, and adjust, as required.