Pull out your tape measure.
Slide both closet doors to one side.
Place the tape measure against the end of the offset rail. Pull it all the way to the beginning of the sliding door. Write down the measurement.
Slide the doors all the way in the opposite direction.
Place the tape measure at the end of the offset nearest the wall, and pull it to the beginning of the sliding door. Write down the measurement.
Add the measurements together to get your total length of the sliding closet roller offset rails.