So the spreadsheet and the Freud website are interested in the maximum chip load. They want to be able to remove the maximum size of chip that you can and all of that is based on the number of flutes, diameter of the bit, and the RPMs. Chip loading is important to supply cooling to the bit. Unfortunately, the X-Carve will have some serious issues moving at those speeds and it will cause the gantry to deflect. This will lead to poor cuts, broken bits (although you’re probably safe from this with a 1/4" bit), and slipped motors/belts.
You can be a lot more aggressive than the recommended 28 IPM, but it all depends on the depth per pass. For a 1/4" 2 flute upcut you could easily go up to 60 IPM with a 0.2" depth per pass. Run it dry without a board in to see how fast that actually makes the machine go. If you’re comfortable with it, give it a go with some scrap wood to make sure it cuts clean.
Experiment in some scrap wood and see what results you get for various settings.