That's a reasonable percentage, but if you require me to pay for crappy service, it won't work more than once. The sad part of that is that the crappy server will never know why I didn't come back.
I'll take it a step further. If I eat in a restaurant that tacks on any % for a gratuity for all parties of any size, it will be the last time I eat there regardless of the level of service.
The ostensible point of tipping is to reward good service and punish bad, however with rare exception, most people still tip the bad waiter or waitress, albeit with perhaps a slight reduction in their normal rate. We all accept that wait staff aren't paid enough in salary and feel compelled to augment their compensation with tips. However to have a place totally take the opportunity to tip based on service out of my hands, and the tips out of the pockets of the wait-staff, will be too much for me.
The custom of tipping is a good deal for the owner of the place, but not so much for the patrons.
If, in my town, restaurants were forced to pay wait-staff at least $15.00 an hour, I would stop tipping. Let the restaurant owner make up for the hiked wages in hiked prices. If the food and service are good, I'll continue to come. It would probably mean a number of restaurants would go out of business and there would be an economic down-turn for the town, but sometimes that's what needed to get idiots out of office.