Yes a 30t 1x front chainring works.
The GRX 400 has these specs:
LOW SPROCKET MAX 36T (42T works)
LOW SPROCKET MIN 30T
TOP SPROCKET MAX 11T
TOP SPROCKET MIN 11T
TOTAL CAPACITY 41
Drive train capacity can be calculated with the following formula: (maximum difference chainrings)+(maximum difference largest and smallest sprocket). E.g. The FC-RX600-2 crank is 46|30, the maximum difference is 16t. Assume an 11-36 cassette, so the maximum difference is 25. Then the capacity of the drivetrain is 16+25=41t.
But if you only have a 1x with an 11-42, your capacity is just the cassette =31.
RD-RX812 - 1x (42 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur
RD-RX810 - 2x (34 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur
RD-RX400 - 2x (36 max tooth)
https://www.jensonusa.com/Shimano-GR...ear-Derailleur
Understood - thanks. Amazing how this can work 11-42 10 speed. Running it 1x won't be a problem?
Edit - let's say i'd like to run a 1x with a 30T chainring up front... is that too much range?
Thanks again.