+-0.8mm Tolerance Issues

I was making some parts and wanted to put a square dato into a square dato hole. It didn't fit. I did some measuring and came up with some odd results. All are in mm and all were supposed to fit together. I made this small shape to test some things

X-Axis Distances
Square Square
Hole Block
4.5 5.4
14.8 15.3
24.4 25.2

Y-Axis Distances
Square Square
Hole Block
4.2 5.4
14.2 15.5
24.1 25.2

I believe the issue is backlash as it doesn't seem to increase with size. I got a friend to print it on his printer which is a Perusa with a 3mm fillament and .35mm nozzle. He printed two and they fit together with no interference. Which makes me annoyed, but it also means I can get this one to work to that standard.

I have a Mendle/Perusa mix: the X axis is from a Mendle and most of the rest of the frame is Perusa. The control board is a RAMPS power by a XBox 360. The extruder is a MixShop extruder assembly with a .5mm nozzle, 3mm fillament.

Any ideas what I can do?

Did your friend use the exact same g code? Or, did your friend sliced the STL him/herself by any chance?

I think (not 100% sure) the dimensional accuracy, especially with inner openings, can be impacted by the slicing software used and also the layer width.

We both use Sli3er. He helped me get mine up and running and I have copied everything software wise from him

Actually, I was dreaming about this last night. I may reduce the fill ratio. I think it may have been caused by the nozzle putting down too much plastic so it spread outwards to the side. I will get back on this.

I mostly fixed my issue
I now have a 0.0-0.1 oversize when outside and 0.3-0.5 undersized when on the inside of a pocket. I replaced the entire X-axis with a new design, something looking like the single piece parts of the Perusa, but using the roller bearings from the Mendle.

Is there a spot in the Sli3er code where I can reduce the perimeter of an object when making G-code? I could put tolerances into a part when designing it, but I will be unable to print off .stl files from other people that are supposed to fit together.