....or just decide how much you're willing to pay in terms of $$ and let the (dealer, internet, whatever) know that that figure is your limit.
If the figure ends up being X% off MSRP, great. If not, you don't care because you've determined what you're willing to pay. If the seller won't match it, great, because without a willing buyer and willing seller, you don't have a market.
If your budget is $50,000, will you pay $60,000 if it is 50% off MSRP? If you're willing to pay that much, then that was your budget, not the original 50,000.
Folks get too wrapped around the axle about getting some magical number off....set your own limits and go from there.