I've always tended to go with up to 10 over the posted limit as my general rule of thumb as to what's safe. But I always look to what's going on around me, my fellow travelers. Never really put numbers to it, but I tend to set my pace such that I'm passin' maybe 85% of the vehicles out there, and allowing the other 15% to pass me. There are exceptions, of course, where I'll wind things out a bit--temporarily--especially if by doing so I can put myself into open areas with few cars around me. I don't mind drivin' in a crowd here and there but, by and large, I'd prefer to have some space and visibility around me.