I wouldn't go any lower than 2 oz. I would suggest as high as 3 oz, but I have limited experience using leaf hops for dry hopping and so I can't comment on any challenges that may present (e.g. excessive absorption of beer, difficulty racking, etc.)
The biggest factor in preventing the grassy/vegetal aroma is contact time. I never, ever dry hop for longer than 5 days. That, in my experience, is the sweet spot for getting great flavor and aroma and preventing that grassy flavor. But again, my experience is almost entirely with pellet hops which have a higher contact surface area, so that sweet spot may be different for leaf. Maybe somebody else can comment on that.