Mixing a wash?

I'm just wondering what is the ratio for mixing just your basic black wash, for the grills and rims. I've seen 1:8 paint to thinner, and 1:20. I'm still a bit confused as to exactly what is a good ratio. Thank you for your help!

I don't think so that there is exactly rate for mixing wash.
Try it on some old models until you are satisfied before you put it on new model.

I used 1:20 with Testors Model Master enamel flat black and mineral spirits - bad luck, all the pigment would flow away down the panel lines and collect in a corner. So I will definitely give your 1:8 ratio a try next time (that is, when I don't get lazy and use a pen :p )