I tend to do the calibration with the firmware changed so you an run without hotend temp set, so change it to 0 instead of 170deg and with the hotend completely off. and run the test multiple times to make sure you get a good number for extruding the right amount cold. Then try again with the hotend up and running.
I generally start with PLA at a high number like 220deg then keep dropping the temp back till it's still flowing well at a lower temp. I find different filaments can vary a fair bit. White PLA from Bilby3D I run at 215 and Blue PLA from Hobbyking at 205.
The olnly other thing I can think of is I have had situations where I was running the spring tension so tight I was deforming the filament so much it was tight in the hotend. Since then I run the tension pretty low and only adjust it tighter if it's necessary.