@yao What you are looking for is called "extrude factor override percent". https://docs.duet3d.com/User_manual/Reference/Gcodes#m221-set-extrude-factor-override-percentage
I don't think there is anyway to adjust that for all extruders concurrently using the web interface. I guess that would a useful feature to have, just like we can extrude from any combination of extruders or "mix".
The only thing I could suggest is that you send M221 from the console but the documentation isn't clear if you can use comma separated values to specify multiple extruders or not - e.g. M221 D0:D1:D2:D3 Snn would do what you want if multiple extruders are acceptable in that command. All I can suggest is that you try it and see.
Otherwise, you could use multiple commands and wrap them in a macro.
e.g.
;macro to set all extruders to 95%
M221 D0 S95
M221 D1 S95
M221 D2 S95
M221 D3 S95.
Rather than create a number of such macros, you could use a global variable to specify the extrusion multiplier.
e.g. M221 D0 S{global.ExtrusionMultiplier} etc...
It's messy I know but offhand, I can't think of anything better. Maybe someone else will....
