The following steps would apply to all unoccupied bases when a defense completes:
- Take the average of 100% and the current % strength of the base.
- Add the new defense at this new percentage.
- Rebalance the base according to the current formula.
Simple example: A PRing completes at your newly freed and 0% defenseless base. Average of 0% and 100% is 50% so you get 2.5 PRings added and no rebalancing is done. The new strength of your base is 50%, if you already had one set of rings it would have been 25% as you'd still get only 2.5 PRings in working order.
The 100% can be modified to a lower value to tone down the strength of completed defenses, including on undamaged base which can mitigate some frustration of having a major defense complete while en route to a target although the odds of these occurrences are for the most part confined to the early game. In those situations I would believe that the health of the game at that stage would be improved as it rewards active and aggressive player fighting for control of a galaxy. Recommended value for this purpose would be no lower than 75%.