I’ve got a quick, super useful tip for you today: Ackerman bargaining. It’s a rule of thumb that could save you quite a bit of money over your lifetime, and all you have to remember is three two-digit numbers.

Whether you love to haggle or can’t stand it, chances are you have to negotiate a price from time to time. Perhaps you’re in the market for a used car, or maybe (like me) you do a lot of your shopping on eBay using the “Make Offer” function. Regardless of the specific context, you’ve probably wondered, “How much below the asking price should I offer?”

In Never Split the Difference: Negotiating As If Your Life Depended On It, former FBI international hostage negotiator Chris Voss lays out a simple formula: Ackerman bargaining. Voss learned the formula from an ex-CIA operative named Mike Ackerman and used it for years in high-stakes ransom negotiations with international kidnappers.

As I’ve written before, Never Split the Difference is well worth your time. Voss spends a few pages on Ackerman bargaining and explains a bit of the theory behind it, but today, I’ll just borrow Voss’ language as he lays out the process itself (the first three points are our focus today).

Set your target price (your goal). Set your first offer at 65 percent of your target price. Calculate three raises of decreasing increments (to 85, 95, and 100 percent). Use lots of empathy and different ways of saying “No” to get the other side to counter before you increase your offer. When calculating the final amount, use precise, non round numbers like, say, $37,893 rather than $38,000. It gives the number credibility and weight. On your final number, throw in a non monetary item (that they probably don’t want) to show you’re at your limit.

I haven’t personally experimented with steps 4-6, but I’ve had repeated success with the percentages laid out in steps 1-3. Just remember the following percentages: 65, 85, 95, 100.

And as you might have guessed, there’s a flip side, too: when you’re selling something, set your asking price at 135% of what you’d accept, and be willing to negotiate down to 115, 105, and finally 100% of your original amount.

Ackerman bargaining is just one more example of how a tiny bit of the right kind of preparation can pay off in a big way. There aren’t too many tactics out there that will save you money with virtually no work at all. Ackerman bargaining will.