IMHO, so called MSRP is largely a myth. The dealer can peg it whatever he wants. Too high he can offer deep discounts 25% means it is overpriced to start with, too low he either gets hard nosed or goes out of business.
I suggest shopping around and compare prices on the model you want. Average them out then make an offer that you think is fair. If he says have a nice day ask him for a counter offer. If outside your range offer to split the difference. There is no magic percentage, it's up to you and your negotiating skills.
I was in outdoor power equipment sales, often my response depended on the customer's attitude. If he/she came in primarily to grind me down I dug my heels in and quoted what I wanted not necessarily what I needed. If the client wanted an argument take it somewhere else, I refused to get beat up just to make a sale. Anybody that treated me like a crook got shown the door. If he/she asked me nicely what I could do for them I would try to be fair to both of us. After all, I had to eat too.If they wavered I would try to sweeten it by offering some extras to throw in.