@ConvenientWoodwork i’d still like to learn about this calibration if you have a tad more detail to share there.
Spoke to soon. I just tried to cut into slab 3.
Because it’s lower the bit length (when tucked all the way in to the router) was not long enough to cut the full depth of the dog hole.
So i pulled it out about another 1/2".
I reprobed z and started again.
Now i’m getting that same extra width along the X axis in the hole. 
How far into the collet are you pushing the bit? There’s usually a mark on the bit that has the insertion limit. But a good rule of thumb is to keep the bit below where the flutes end. That way any chips moving up the bit flutes will exit without jamming up into the collet.
Also, when replacing the bits you’ll want to tap the collet to make sure any sawdust drops out of it.
In VCarve you can add a finishing pass to your holes - I’d set that so you have something like .05 as the finishing path (even .1"). That lets it hog out the hole slightly undersized and then will make a final skin cut to take off that last tiny bit to make the hole the right size.
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This would be your “calibration” program.
Backlash can be induced by being specific with your cutting path creation. By approaching a cut from the left, from the right (and/or top and bottom if you want to check the Y axis) and overshooting, you should be able to see the backlash of it exists.
For example, assume your origin is at machine home. The intent is to cut a small tic mark as @Aiph5u suggested every 20mm along Y+20. The first tic is approached from X0Y20 to X20Y20. The next tic is approached from X(max)Y20 to X40Y20. The next tic is approached from X0Y20 to X62Y20 and finally to X60Y20. Theoretically, all your marks should be exactly 20mm apart. If you have backlash they won’t be consistently spaced. If they are consistent but not exactly 20mm, your step/rev needs tweaked.
In Fusion these toolpaths can be accomplished with a sketch made visible in the manufacturing workspace. Use a trace cut path on the sketch lines.
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You may check on the effect of the pull from your vacuum hose line. The pull will be stronger as the router moves to the right. Any ball screw play would be affected by this force.
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Confirmed that drag from vacuum hose was not an issue. Thank you!