Correct way to work numbers when buying a new vehicle

From : chris

Q: what is the correct way to figure out the amount you need to finance with all these factors + truck price 22000 + tax 1870 + title registration 300 - rebates 3500 - cash down 1000 - trade in value 18000 + payoff for trade in 13500 apr% 3.5 term months 60 amount financed this is edmunds web site where you just punch everything in http//www.edmunds.com/new/2004/dodge/rampickup1500/100289755/calculator.htm ltid=edmunds.n.pcalc.calculator.introcalc.1.* the above is just an example. but would i take the $4500 difference between trade in and payoff and subtract it from the truck price before taxes or can i just take the trade in price and deduct that from the truck price before taxes etc. what is the correct sequence to figure the amount financed from this example people i have talked to around here say that the trade can be subtracted before taxes just like the small print says under my monthly payment on the web page. also if its not too much trouble how do you figure it out on paper if i didnt have the site to punch in all the numbers.thanks .