Shimano GRX RX 815 Series Dealer's Manual page 118

ADJUSTMENT
Adjusting the rear derailleur
3.
Adjust the end adjustment screw.
Bring the guide pulley close to the gear to a position where the chain does not jam.
Largest sprocket
4.
Check that the chain does not jam even at the smallest sprocket.
If there is some slack in the chain, adjust the end adjustment screw to remove the slack
from the chain.
If the frame interferes with the rear derailleur, adjust by turning the end adjustment
screw until they do not interfere.
End adjustment screw
B
A
A
B
B
118
*
A
Smallest sprocket
* End adjustment screw