The inboard is determined by where you set the collar on the sleeve. Here are some basic recommendations for both sculls and sweep oars.
Sculls
Measure the spread of your boat (the distance between the pins that hold the oarlocks). Take half of this dimension and add 6–8 cm. This will give you a reasonable inboard dimension. Measure this distance from the end of the handle and position the collars at that point. Tighten the fasteners evenly and firmly.
Sweeps
The spread of a sweep boat is the horizontal distance from the center of the boat to the center of the pin that holds the oarlock. To this dimension, add 29–31 centimeters to come up with a reasonable inboard dimension. Measure this distance from the end of the handle, and position the collars at that point. Tighten the fasteners evenly and firmly.
Bob Beeman from the Concept2 Oar Shop demonstrates how to set inboard and finish installing collars on your Concept2 Oars.
See page 6 of the Oar Manual for Collar Installation and Positioning