I'm with Splasha LOLOL..Charts are all good and that one no doubt is handy for many but it is really simple to mix up for greyscale..Start with half a cup of white, add one drop of black...test spray...Is it dark enough? add another drop of black if not, no more if happy LOL..
But this is the way I do it...
Make a mid grey (A mid grey is easily judged for most, oh yer make a shizload, bout 200ml), spilt it in 3 and adjust 2 of them, one with about the same volume of white, bout 1-1, the other with 1-1 black, again 3 distinct grey shades..Reduce then to your desire..Making a decent amount is very important, grey believe it or not is bloody hard to match perfectly unless ya wrote down ya mix..It doesn't matter if ya make two much, often I'll use a full 500ml bottle of white to do this..When ya have ya 3 main shades, put them in their own paint bottle and you'll likely have enough for a few decent sized works..
So there is now quite a fair mix of 3 distinct shades of grey..When spraying these individual shades, through speed, trigger control and intensity of the opaque paint control IE 0% through to 100% you have now in theory created at least 300 different shades of grey..
Mixing too many shades can get a little confusing, like which one did I use again for that and having such a minute difference between them IE 10% wnt look a hell of a lot different to 20% in the jar seems for me a little overkill..
Just mix your main 3, its easy once ya get used to what a light, medium and dark grey looks like and that's pretty easily judged, use intensity and build up then from these main 3 to create a myriad of grey shades..Good luck..