Ramps 1.4 1.8 stepper motor extrusion calculation
According to the description, the drive gear you have has a 10.8mm diameter. This means that (in the ideal case) one full rotation of the drive gear will advance a length of filament equal to its circumference, which is pi x 10.8mm or approximately 33.93mm.

Your motor rotates 1.8 degrees per step, so it takes 360 / 1.8 = 200 steps for a full rotation. Since you are using 16x microstepping, this is multiplied to 200 x 16 = 3200 steps.

You thus end up with a steps per mm value of 3200 / 33.93 = 94.31 steps/mm.

You might need to calibrate this further, for instance by extruding a set length of filament (e.g. 100mm) and measuring how much is actually extruded, and then compensating the steps/mm value to get you closer to the desired 100mm. A simple way to measure this is to put a mark on your filament at 150mm from the extruder, and then (after extruding 100mm) measure how close the mark is to the extruder (which should be 50mm) However, this theoretically computed value should be a good starting point. Note that the speed you do this test at should be close to your normal printing speed, since extruding at a much higher (resp. lower) speed will falsely lead you to believe you are underextruding (resp. overextruding).