He doesn't. He and his agent have educated guesses. But it's still a year and a half out before they can "know".
You've seen those hurricane cone maps?
It's kind of like that, except probably with even more variance, and without a certainty of "weakening" like a storm will do as its geographic track plays out.
We're at Wednesday AM right now. We're trying to pick a price that shuts the door on him finding out where the Monday AM location will be. Some players might accept the conservative offer, to reduce all the risk. Other players are said to "bet on themselves", and you will have to offer pretty close to the maximum in that cone of probabilities, to shut that door for them.
5 years for $80M is $16M a year. That is Kimbrel/Chapman territory - top relievers, but starters get more. Money like Bauer and Cole got may not be achievable. But I'm thinking $20M for 7 years is what's needed to get him to say, "OK, never mind about the Yankees and free agency." 1.5 years away, $140M will get his attention, $80M will not.