Welcome to the SeeMeCNC Community

I've been trying o print an endcap for a square PVC fencepost. The cap will fit inside the post and I've measured it to be a 96mm square. When I print the piece, it comes out 96 mm in the X dimension but 97 mm in the Y dimension. This is a on a V1 Max. I've searched the forums a lot to find what I should change to adjust the Y dimension smaller by about 1% but haven't found how to do that yet. So far, I've tried:

1) Updating to the latest firmware and re-leveling the bed (including the horizontal radius) using the directions in the manual.

2) Re-squaring the towers and adjusting their heights to make sure they all match (and then re-leveling it again).

Is this something that can be fixed in the firmware or is it an error rate that I'll have to live with? Thanks.

Try a different slicer to slice the part, make sure your belts are all snug and hopefully others will step in with their opinions.

I had about a 5% problem in Y with my MAX v2 and it turned out to be that the bolt used to hold the idler on the 'Y' tower, it was slanted (lower on one side than the other), this led to the belt rubbing on idler and I think caused my problem. In correcting this I also would have adjusted the tension, so its quite possible Eaglezsoar's answer was what fixed my problem too. Although, I'm still about 0.5% off from dead on, but that's close enough for me.

The short version is that you can't adjust for just the X or Y dimension independently in the Firmware.
You can change the overall X/Y scaling to some extent, but not X or Y.
The odds are it's a mechanical issue, belts are worth checking, backlash at one tower could cause it, the other possibility is that one tower is not equidistant from the build center.
I don't know if OpenDact will accommodate for tower radius offsets, but it might be worth a shot.
The only other solution is to measure everything very carefully, and it's not a simple dimension to measure accurately.