1x GRX chain sizing with 42T cassette I'm trying to convert a bike to a Shimano GRX 1x setup using the following components: RD-RX812, CS-M8000 (11-42T), CN-HG701-11 (and FC-RX810-1, 42T). The problem I'm stuck at is when shifted to the easiest gear (i.e. the 42T), the topmost rear derailleur jockey wheel sits too close to the cassette, causing the chain to not run smoothly from the cassette through the rear derailleur (see picture).



Can you help figure out what I am doing wrong here? Is the chain too long? I have seen images of GRX setups where the lower jockey wheel is pulled much more forward when in the biggest chainring (example). At the same time I am a bit reluctant to cut my chain, as the current length is exactly what Shimano recommends (adding 4-6 links to the length when wrapped around the biggest rings but not through the derailleur). Any pointers are most appreciated.
You need to adjust the derailleur B-screw. That adjusts the distance between the top jockey wheel and sprockets. Winding in the B-screw will move the jockey wheel away from the sprocket. The gap should be ~5mm.

See https://www.parktool.com/blog/repair-help/rear-derailleur-adjustment. The last section deals with B-screw adjustment.

You are correct that you should not change the chain length.