Tower differential measurement

We are having issues with implementing APC programming due to some tower differential measurments. I have some concerns with the fact we are using the tower bottoms and the overhead line. We have made some improvements as far as tubing and meter location and have not noticed a change. They in fact seem to have gotten worse. So that being said do any of you have some best practices or maybe a detail drawing you could share, or just some " I have seen this" instances?