What is the formula for Unarmored Movement's bonus?

This is more of an explanation on how to find the formula:

We have here is a set of points where $x$ is the level and $y$ is the unarmored movement bonus. This gives us the set of points $S = {(2,10), space (6,15), space (10,20), space (14, 25), space (18,30)}$.

Fortunately all of these points lie on the line $y = 1.25x + 7.5$ which can be rewritten as $y = frac{5}{4}(x + 6)$. Unfortunately just using this equation would get us weird things like having 13.75 bonus movement at level 5 and so we have to remove the parts of the line that are giving us values where we don't want them.

Basically we want to make levels 2-5 identical, levels 6-9 identical, and so forth. These are all intervals of length four so we can use the floor function with division by 4 to get exactly what we need:

$ y = 5 times lfloor {frac {x+6}{4} } rfloor $

You can obtain the correct Unarmored Movement for levels when the character has that feature using math.floor((MonkLevel+6)/4)*5. This gives correct unarmored movement for levels 2-20; at level 1 this would give 5 ft. of unarmored movement, but monks don't have the feature at level 1, so you're fine =)

If you're dead-set on inputting the formula at creation and never looking back, just go for if(MonkLevel>1, math.floor((MonkLevel+6)/4)*5, 0).

DiceCloud's helper equations page has other examples, but I find their suggested Unarmored Movement formula overly-wrought, being composed of many logical tests rather than utilizing the fact that the movement rate changes every 4th level.