After some discussion about how to calibrate the extruder esteps, I want to share another way to fine-tune them. This version is a derivate of Triffid Hunter’s calibration guide for E-steps fine tuning.
In the end the only thing that counts is a perfect top surface finish. Thats why in the linked guide he is suggesting to watch the top layers for small gaps and make small adjustments to the esteps until it’s perfect.
The problem with this approach: It’s very hard to see if there are very tiny gaps or if they are already closed. You can use a flashlight under a particular angle to the part to see more details, but even with this it’s sometimes hard to decide.
The perfect way would be to use one of this USB microscopes, but not everyone has such a device. But many people own a scanner 
If you use the highest setting for the dpi value, you get a nice magnification, about 13x with my one. Thats enough to compare prints easily.
Have a look on my example picture in the attachment:
The blue part was one of my early attemts to calibrate esteps moth ago. Their are some gaps in the top surface, a clear sign of under extrusion. The red one has a smooth top surface, the value for esteps is quite good here.
You can use this technique also to do precise measurements if the part dimensions, using a picture manipulating program of your choice and select the measurement tool. I’m using Gimp as an example, it’s even possible to measure the line with 
If you want to try it, follow Triffids steps but print a flat square with some mm height. Start at a low extrusion value and slowly approach the perfect value. Have a lot of fun 
