New car reasonable discount

Domestic 20% plus
Toyota/Honda probably 15%.

This is a car right? Trucks would be higher.

Look into the will cover method when buying a vehicle

You ask a complicated question. There as so many ways for dealers of the various makes to build profit margins within each vehicle line, PLUS there are many factory-to-dealer incentives during certain times of the year and on certain models. Watch for the additional "price sticker" for dealer add-ons and "prep fees".
Finally, if you've got a trade-in, it's a whole new ball game!......did I mention factory-to-dealer model change allowances?
A tip from a guy who was in the business for 30 years:
Go get an NADA hand book that gives the retail/wholesale prices for all the makes and models plus fgt charges. 1) find the car you want 2) Use the factory sticker to add up all the options, base price and fgt. from a wholesale basis (put dealer installed options to the side for now) 3) Use google to find if there are any special incentives on your particular model at this time, THEN, 5) if you have a trade-in, get a "black book"price( you can take your car to many dealers and they'll give you a price that they'll buy the vehicle for...I'd do a min of 3 dealers..and you'll get an idea)
Finally, the dealer added "options" are where the dealers REALLY run the margin of profit up, usually so they can show the customer a larger discount(don't blame the dealers for this, because the manufacturers and unions forced them to have to go this route). There is no rule-of -thumb for these profit margins so you're on your own here with negotiations(tell'em to take the options or get you one without the options added )
This is about all the help I can give you at this point. I sold my dealership and have been out of the business for nearly 20 years, so I'm sure a few thing have changed.
If all else fails, there are car-buying services, or maybe you know someone who's been management at a dealership for a while and has a good business head.
GOOD LUCK!

Hell, I drive an old clunker now with 150,000 mile on it (never invest in a depreciating asset if you can help it!), but my wife and daughter drive pretty nice cars, so when it's time for me to get them a new one, I go have a good time doing it!

Also be sure to check the car manufacturers website for any special offers. Its uncommon but less honest dealerships may pocket your rebate and not tell the customer.

There are websites like Truecar that are supposed to tell you what others paid in your area although I'm not sure I trust them.