Yes, towers22, that is an approximate rule that refers to when hydrangeas begin to develop new buds and it applies to hydrangeas -like yours- which bloom on old wood. You could start pruning any day now. Below is a link with more pruning information. You should try to do some pruning every five years or so to revitalize the plants. Personally, I cannot remember the last time I pruned my hydrangeas (too many plants, too little memory) so I prune after noticing unpruned plants that suddendly have weak bloomage which I cannot explain.

But I have one concern. Why did the plant bloom from the interior only? I would have expected weak bloomage throughout after years of neglect but not interior bloomage only.

So what can you do about that? If pruning now does not restore bloomage throughout the plant in 2008, you may need some winter protection techniques applied so winter does not kill the flower buds. If you are already using winter protection techniques, consider enhancing their coverage as some winter cold may be getting through and killing buds at the end of the stems.

Good luck pruning,
Luis

Here is a link that might be useful: Pruning Oakleafs Using Method One