Board Thread:The Last Sovereign Discussion/@comment-89.180.58.2-20180513105708/@comment-35006067-20180513112429

According to this post, you need a Headquarters Magic State of 30 (or more) to get the best evaluation, but 20 is enough to be considered a great defense level.

You get +10 for the Base Defense Research Project, and about +5 per shielding upgrade you invest into. See there for details.