First you need a way to measure 100mm accurately. The best would be a digital caliper like this one. Neiko 01407A Electronic Digital Caliper but any digital caliper will do. Unload your filament. Cut it square. Remove the bowden tube from the extruder. Align the top of the filament exactly with the top of the bowden fitting. Now send the following commands to the printer:
G91 M302 S0 G0 E100 F150
The G91 command sets relative mode. M302 S0 allows extrusion even though the hotend is not hot. The G0 command tells the extruder to extrude 100mm at a reasonable rate.
Now, using repetier-host, display the number of steps per millimeter for the extruder. (Alt-e will call up the display of the EPROM,) Call that Sold. Measure the filament you got. Call that Fold. Run thus formula:
Snew = Sold*(100/Fold)
Put the value you got for Snew into your EPROM display as steps per millimeter, and then repeat the test by pulling the filament back and extruding again.
If it measures 100mm as close as you can measure it, you are done. If not, plug everything into the formula (using your new steps per millimeter as Sold) and try again.