After a couple failed attempts to print parts the are supposed to be mated together (they didn't fit), I realized that I don't fully understand how much error my printer has in each axis. So I designed these cubes which can be used to come up with an error profile for your printer.

For each print setting you want to test (speed, number of shells, layer height, etc), print this out, and use a caliper to take 5 measurements on each of the 3 pieces and compare to the expected sizes. Measure:

X-inner (hole) width
X-outer (peg) width
Y-inner (hole) width
Y-outer (peg) width
Z height

I expect that if you tell the printer to print a cube with X-axis size S, the actual printed size, T, will have both constant error, c (like 0.28 mm), and multiplicative error, m (like 4.6%). In otherwords:

T = m*S + c

Therefore, you technically only need two pieces to get two (S,T) pairs you can plug in and solve for m and c. I print three, because I want to do the calculation between each pair and confirm the answers match (approximately)

Notes:
-- The X and Y dimensions of the cube are the same size, but only X is marked in the diagrams.
-- My printer does have different errors in X and Y, so be sure to measure and compute them separately
-- The notches in the upper-right corner are simply to maintain XY-orientation after you've removed them from the build platform. Without it, you may not remember which direction was X and which was Y.
-- Technically, you could probably figure out inner sizes just from the outer sizes, but I wanted more-direct measurements, since part of the reason I'm doing this is so I can make pegs and slots that fit on the first try.
-- I purchased this caliper on Amazon and love it. Totally worth $30 (and appears to be fairly accurate despite the price)

TODO: Will make a simple python script that will let you type in your measurements, and will estimate "m" and "c" for you. Then, let you type in the target size of an object, and it will tell you how big to model it (or scale it, if already made), to get the target size.