Hi Jennifer, I've never used the R.L silver paint, but I have used glazes for technique painting. Ralph Lauren makes a glaze, probably best to stick with their glaze for their specialized paint. To save yourself a lot of agony, I'd highly recommend practicing on a piece of scrap drywall that has been painted with your gray base. (preferably prime it first for a "true" effect). I've found what you anticipate your technique will look like, and how it actually turns out, are often entirely different :-).
I would not use any more than 50% glaze, or most likely you'll lose the effect of the silver paint. I would start with a 50/50 mix, it will give you a good transparency and allow your gray to show through. If that's too thin, the standard ratio I use is 1 part glaze to 3 parts paint. Using glaze will not only make your silver transparent, it will allow you more working time. Best wishes, Kathy