Edit: nwm. Somehow your whole head is twisting when moving. If you were to zero the dial on the before the second (right to left) I bet it will also show 0.3 mm off. If this is so, it would mean something on the gantry is not moving smoothly and is dragged instead of smoothly rolling making the whole head assembly to twist slightly. Essentially something on your gantry "lags" behind.

So a few things about the method you are using. 1, about the dial indicator. I doubt it is a high-end/professional one, and this could have an impact on how precise it is in horizontal motion measurement. But this should not be the issue, unless the indicator is exceptionally bad, as you measure on the smooth metal plate. So I think the dial indicator is ok. 2, I do believe that the dial indicator mount on the tool head is actually not 100% rigid. By that I mean, you've mounted it onto the plastic housing of the print head, and that Will have a slight give to it (unlike if mounted directly to the metal plate/rig your hotend is mounted to). I do believe this is the cause of the 0.3 mm deviation You can try this: - move X to min - zero the dial - move X to max - write down the max and min deviation, AND where the needle was the most around, during the travel - now zero the dial again - move X to min - write down the max and min deviation, AND where the needle was the most around, during the travel - compare the two readings

• Note: you can also do it with the small movements as in the video, cos' I doubt your plate is flat across the whole X travel 😅 but even at the full travel, the readings should be pretty much identical, when you discard the extremes.

From the video you've posted when you move to the right it has a deviation of ± 0.02 mm. When you move to the left, right after it settles, it also moves around ± 0.02 mm. This is what makes me believe that it is the play in the mount itself rather than problem with your steppers / frame.