Rear derailleur shifting

[OakLeaf]

How did you cut the housing ... did you make sure to remove all burrs from the ends? if you have ferrules or barrel adjusters, is the housing inserted firmly with no wobble? is the ball end well seated in the shifter? Are you certain the housings are correctly routed?

The B screw aligns the derailleur body to move the top jockey wheel toward/away from the cassette. In your biggest cog there should only be a small amount of clearance. Still, one tooth smaller shouldn't keep it from shifting without an adjustment, just might be a little noisy. Same with chain length.