How did Hamilton figure out the pattern?

At about an hour and four minutes into the movie, Texas Ranger Marcus Hamilton (Jeff Bridges) explicitly states his reasoning to his partner, Alberto Parker (Gil Birmingham):

He deduces the next likely location while considering:

• Vernon, TX (population ~10,000) is a "fair sized town" with a Texas Statewide Bank not a "Midlands Bank."
• the size of the towns and banks which the brothers have robbed so far
• that there are only seven Texas Midland banks total
• his hunch that the robbers aren't tweakers (because they were smart and only took loose bills, small denominations, "no ink pack" and no "hundreds")
• that they hit banks early in the morning ("A little early in the morning for tweakers")
• his hunch that they have a plan to raise a certain amount
• he also figures that because Tanner Howard (Ben Foster) robbed one bank alone, it was not part of the plan.

Because he realizes the robbery in Vernon was an aberration, Hamilton is able to see the pattern. As he states earlier in the film, he is confident that they are not just some meth heads (like Parker suggests while they're driving to investigate the Vernon robbery) and instead these robbers have a goal and a plan how to get it.

Hamilton: Well, these boys, they ain't done yet, I'll tell you that.

Parker: How come?

Hamilton: Well, they're patient. They're just sticking to the drawers, not taking the hundreds - that's the banks money, we can trace that. They're trying to raise a certain amount, that's my guess. It's gonna take a few banks to get there.

He doesn't need to know the goal, and figures that the plan is to rob Texas Midlands Banks in small towns. He then considers the seven Texas Midland Banks in the area and (excluding Coleman and Ft. Worth) the choice is between Post or Childress. He deduces that the one with the smallest population is the obvious next choice.

"The only branch that fits the bill is in Post"
-Marcus Hamilton (Jeff Bridges)

In the film's narrative, knowing the area isn't required for accepting the idea that Hamilton knows what he's doing (despite Parker's "reluctance" to oblige his deductions), but looking at a map and knowing the town populations might help with understanding Hamilton's thinking:

"They aren't gonna mess with a bank in Childress, that's a fairly decent sized town"
-Marcus Hamilton (Jeff Bridges)

This map shows the general area West of Fort Worth, TX (pop. ~800,000) where the film takes place:

• Jayton, TX (pop. ~500) - CLOSED
• Archer City, TX (pop. 1,750) - ROBBED
• Albany, TX (pop. ~2,000) - ROBBED
• Coleman, TX (pop. ~4,500) - THE TOWN THEY ARE IN
• Post, TX (pop. ~5,200)
• Childress, TX (pop. ~6,000)
• Vernon, TX (pop. ~10,000) ROBBED
• Ft. Worth, TX (pop 800k) MAIN BRANCH

Hamilton: I think I got this figured. The first two banks, they were Texas Midlands Banks. Alright, there's seven branches altogether. The main branch is in Fort Worth. They're not gonna mess with that. Alright, they hit the branch in Albany, they hit the one in Archer City, then there's the one here.

Parker: Which they did not hit

...

H: There's the one here. Then there's the one in Childress. There's the one in Jayton.

P: That one's closed

...

H: That's my point: Jayton is closed. That just leaves Post. They aren't gonna mess with a bank in Childress. That's a fairly decent-sized town.

P: Branch in Vernon's wasn't Midlands and they hit that.

H: One of them did.

Post is "the only branch that fits the bill" because Hamilton has figured out that they are focusing on small Texas Midlands Banks by presuming the Vernon robbery was a fluke.

Bank plot, Hell or High Water.

Marcus Hamilton and Alberto Parker are law men. Toby and Tanner Howard are bank robbers.

They key to keeping on track with the plot to do with the banks is in just a few conversations. Two being at around the same time before Post is robbed and the other conversation being relatively near the start of the movie.

Conversation 1. After Alberto Parker informs Marcus Hamilton about the bank robberies that happen at the start of the film, Marcus and Alberto head to the bank where the manager was hit in the nose, whilst at this branch: Marcus looks at the scene thinks of the amounts taken so far and states in his opinion that the bank robbers are trying to raise a certain amount and it will take a few banks to get there. “They’re trying to raise a certain amount, that's my guess” - Marcus Hamilton

Conversation 2. After Tanner Howard and Toby Howard get to the bank branch that has been closed down that they were not aware of, they drive to rob another small branch in Coleman. On the way Tanner gets Toby to pull over and explains why hitting Coleman is a waste of time.

“There’s only one teller in Coleman and whatever we get won't be enough” - Tanner Howard

Tanner goes on to explain things and ultimately convinces Toby to head to post, one bank and scrap the plan of two banks, Coleman and the closed branch.

Conversation 3. We know this conversation happens in Coleman as there is a scene out the front of the T-bone steak cafe that says Coleman Hotel.

Marcus Hamilton explains to Alberto Parker

“The only branch that fits the bill is in Post” - Marcus Hamilton

The explanation is in the form of a one sided conversation that ultimately explains the closed branch. Closed. Marcus has deduced that with Coleman (where he is in wait) combined with the no cash (of course) from the closed branch the only other branch aside from the very large one (which he knows they won't hit as it is explained earlier about the amounts of cash being taken) is Post.