I find that the estimated size at maturity will be off in places where the growing season is long (say, in the South). Since they do not stop growing when they reach 10 years of age, it may get taller/wider afterwards. If it does then prune accordingly. Enjoy W/D. I do not have it here but it is a very nice dark red!

The height on labels is customarily the height at 10 years of age. Very few happy plants stop growing when they are 10 years old. If you go to Scotland where many plants are 100 years old, you find many rhododendrons that are more than 20 foot tall. The saving grace is that they grow slowly and are easy to prune to keep near the "10 year old" height.

Here is a link that might be useful: How to care for rhododendrons and azaleas.