To calibrate E steps with the hotend attached you should extrude as slowly as possible to compensate for the resistance of the nozzle. The following commands at the OctoPrint terminal will achieve this:
G21 ; set to millimeters
G90 ; use absolute positioning
T0 ; select extruder 0
G92 E0 ; zero the extrusion distance
G1 E100 F50 ; extrude 100mm very slowly
G92 E0 ; zero the extrusion distance