Help me calibrate my extruder.

So my 3D printer is a replica of a makerbot and it prints in .x3g through ReplicatorG.

After a lot of use my printer is over extruding and I need to fix this as soon as possible. I have been researching as much as possible but as my 3D printer has no documentation I found the following link:

http://www.instructables.com/id/How-...ur-3d-Printer/

However, once I plugged my printer into my computer I do not have the option to extrude a set amount of plastic and calibrate the extruder through ReplicatorG. Can anyone point me in the correct direction.

I do use Slic3r when generating G code and they do have an "Extrusion Multiplier". I would like to not guess and check using this method as I need it as close to accurate as possible. Since I print parts that are used within rockets I build.

Thanks.