I think 6-8 inches would be good. That would give you a nice curve without losing much counter.

I would go from 15" (the minimum recommended overhang for counter height seating) to 19" in the middle. I would also see if you can lose those legs, or you will only really have room for 3 seats (seating is 24"W/per person for a squeeze, 30" to be comfortable for adults - you have 21"/per person).

If you keep the legs and are OK with only 3 seats, I think that aesthetically, having the ends at 13" and flaring to 19" in the middle would be most pleasing.

Another way to look at it is how much you want the two diners on the ends to be able to see each other for conversation. If this will be an area for frequent gathering, then I'd go all the way to the 19" overhang at the center and curve it right back to the legs on the sides. It still won't curve that much over 96", but will make a big difference to those end stools, and for the people in them being easily included in the conversation. OTOH it makes it harder to put a cranky spouse/child "out of the picture" as needed :)! Of course we love them, but they do all have their days....