His floor: dropping out of the primaries after failing to gain any traction.
His floor/ceiling if he somehow gets the nomination is the same as any Republican. Sorry folks, he's not winning MA.
Even if Charlie Baker didn't win MA, he could possibly get more than 40% of the vote, and being from MA would probably help with winning New Hampshire. Warren vs. Baker would be a fun campaign- I wonder who would win (and what the MA margins would be, since Baker is wicked popular and Warren isn't).
You realize that anything Baker does to win the national Republican primary would remove any crossover appeal he has in Massachusetts? The only reason Baker is popular is because he hasn't done anything noteworthy.
It would be Mitt Romney all over again.