You don't need an adjustable VR. What you need, as already said, is to get solid "no dropped" voltage to the VR.
Hook a meter one lead as close to the VR IGN terminal as you can gte and the other on battery POS. You are hoping for a very low voltage, maybe less than .3 v (3/10 of one volt) Whatever above that you read is ADDED to the VR set point, IE if you read .8V, and the VR is correctly at 14, the battery will "run" at 14.8.........example.
