Advice on price negotiation for buying a used car with cash?

So I wanted to buy this 02 Buick from a used car dealer I trust, I've bought 2 other cars from them in the past. They were asking $6995 for it and they came down to $6100. So I told them I'd be in tomorrow night to buy it. But now my dad is making me have 2nd thoughts, he says they would have come down to at least the high $5k's, if I had negotiated better. Its only a few hundred dollars, but I do want to get the best deal & lowest price possible. I had already checked Kelly Blue Book, the suggested retail value is $6560. But now I checked Edmunds True Market Value pricing, which gives it a retail price of only $5779. Which one whould I go by? Should I refuse to buy it unless they come down more to $5800 or so?