Y Axis skips on contour cut

[rhwithers (Ron W.)]

Last night I was doing some contour cutting and getting a few slips in the Y-Axis direction. I was able to observe one of these. My speeds seem conservative but here they are
1/4" Endmill
DOC .125
Plunge 25
Speed 40
RPM 18K

Possibly the belts are too loose and need some adjustment (photo attached)? How tight should the be. I have read this article Measuring belt tension, squaring and calibration - #9 by Julien but for some reason its just not clicking with me at the moment.

[Julien]

You don’t need to bother with the precise adjustment techniques really.
But by the look of your pictures and assuming you were only pulling lightly on the belt, I would think you can use some more tightening indeed. I like to have them tight enough that I can easily slip one finger under the belt but not quite two, if that makes sense.
Slipping can be due to (and is often likely due to) loose pulleys also, but unlikely in the Y direction