Help with binder sizing?

[ScrubDub]

Hello! FTM here. I want to buy a binder from underworks, but my bust is 31inch. This is the highest number it’ll go up to on size S (something like: 27-31) and the the number before lowest on M (32-36?) I don’t want it to be (too) painful, but I also want it to flatten the most it can. I’m thinking of just getting size S. Does anyone recommend anything?
 

here’s the link: 

https://www.underworks.com/econo-high-power-compression-chest-binder-top

[TrIIIy]

When I bought a binder from gc2b, their instructions on sizing said to go up a size if you were right on the line. I would imagine that is a general rule of thumb.