I agree that I don't think you can look for a standard percent off of MSRP as it will vary widely depending on the dealer, time of year, time of month, inventory availability (like now), etc. At the end of the day, it is a business transaction. The dealer wants to sell vehicles and you want to buy a vehicle. They will not sell the vehicle if the price is below what they determine they need to get for it and you should have a price established that is the maximum you will pay for the particular vehicle. That is where you will need to do your research to determine what a reasonable price that you are comfortable with. It is a little easier to research new vehicle values as opposed to used vehicles because you will likely never know how much a dealer is into a used vehicle.
Personally, when I negotiate with a dealer, I only talk out-the-door price that includes tax, tag, title, fees, etc (I don't typically trade so I'm not dealing with trade values and allowances). My simple reasoning is that it is the final price I'm concerned with and have to pay. I don't really care what number goes in which category. I start at or below invoice depending on incentives/promotions and have a number that I will not go above. If the dealer won't deal, I go somewhere else. They aren't selling unicorns; someone else will have what you want and will deal. I bought a car one time that the first dealer didn't want to deal on and the second dealer did. The second dealer did a dealer trade to get the actual vehicle I had looked at and driven at the first dealership. The last vehicle we purchased was negotiated completely via email. I didn't set foot in the dealership until we had a number agreed on and I went to test drive and sign the papers. I was in and out in less than an hour. I've never had to go to or contact more than three dealerships to get a price I was happy with.
Of course, as you know, now is probably the worst possible time to buy a vehicle (new or used) because of the market. We are currently looking at cars, but are making due with what we have until the market correction occurs. Hopefully it will ease in the next few months and certainly by next summer.