Charlie's right -follow the manufacturer's suggestions for thinning, and do it "by eye". I also prefer Floquil and generally find 50/50 works well, but so does 40/60 or 60/40.
:-D:-D For weathering, I use as much as 90% thinner, depending on the effect I want.
I have used syringes for measuring water-based paints, but lacquer-based paints such as Floquil will attack the rubber-like material on the end of the plunger, causing it to swell even before you can squeeze out the paint. wall1:-D Too much trouble even for water-based paints, though, as it just means more stuff to clean.
Wayne