IIRC, when you open up the battery compartment of the 308 (I had one some years ago so I forget exactly), there should be a little dial that you can set with an eyeglasses screwdriver to calibrate the meter. You would turn it one direction to adjust for underexposure, the other for overexposure.
When you say it consistently underexposes, by how much does it vary from the meter in your D70?
Make certain that you are orienting the meter in a purely vertical orientation, perpendicular to the ground, or at least parallel to your film plane. It is a great little meter and very easy to work with.