SRAM force rear mech B screw

My bike has a Sram Force Cx1 rear mech. The jockey wheel is 15mm from the largest cog on the cassette. The B screw is currently all the way out so I cannot decrease the gap to the specified 6mm SRAM suggest. Shifting is currently not great.
Does anyone know what determines where the adjustment of the B screw 'sits'?
Would a longer chain move the mech round and make is so I can close the gap?
Would a shorter chain move the mech round and make it so I can close the gap?
Is it dependant on the hanger?

Any words if wisdom would be greatly appreciated as I cannot find answers anywhere else.

Bike info:
Vitus Energie CRX 2019. (Cyclocross bike)
SRAM force Cx1 groupset throughout.
38t chain ring. 11-32 cassette.