PSU ratio depends on the specific PSU, as some do better at higher wattages and some lower wattages for best efficiency. We'd need the specific model to have an idea, but generally around 80 is a good spot. Going near max is ok as most PSUs are 80+ certified and will stay above that for the most part.
Any GPU I use will be 80+ Gold rated or better.
EDIT: (to add) I don't have the GPU yet, I will use EVGA, Corsair, Thermaltake, Seasonic etc at the above rating. Just what ever I find at the best deal when I buy. Just wanted to make sure that 80% usage was a good place to be for this.
