The next step on the calibration journey is the extruder calibration. This is a fairly straight forward and easy procedure. It has been described many many times on the internet so I am going to be short and only tell you what I manage to get working.
First of all, you can find one of the best calibration procedure here. It’s most of the best I know, it is simple and works !
I followed the instruction, disconnect the PTFE pipe from the hot end, measure with a vernier caliper the length, extracted a certain distance, measured again. In my case, the value in the configuration was good enough and I did not have to get anything change.
If you want a more accurate version of the measurement, I can only recommend the this project on Thingiverse. It is really simple to print and to use. On top of that it will get you a top accurate measurement and therefore a top accurate e steps !
Now even if this step sound really simple, it is really important you get it right. The more accurate you are the better you prints will be. Of course, for each filament type you can use the extrusion factor to correct the extrusion but you do this step really nice, you will see that you will have really or no correction to do.
When you are done, you can then check the fine tuning of the extruder. To be honest, I did not do this step. Instead, I use each print I do to fine tune the extruder by adapting the extrusion factor in Duet configuration. I like to have these kind of configuration in a filament setting, that I load every time I print something with this type of filament.
After a few trials ( and errors ) you will get a top surface, as good as it can get on a FDM. Happy printing !