Nance, I don't have a specific number for you. 80% is a great maintenance number, but keep in mind that there is no such thing as 100% of your maximum heart rate... if you're at that number, and your heart lets you go above it... then you've set a new maximum! I would just go with what feels good for you, as you are doing. Spending some time at 85 or even 95% can get you something, too. If you were to do an interval workout like Imax, a range of 85% for the blasts and 75% for the recoveries would work out well, too.
The more fit you become, the lower your RHR will be. The older you get, the lower your max will be. It sounds like you keep an eye on both! I was simply curious because it seemed from your earlier posts in this thread that 80, and only 80, was where you wanted to be. I simply wanted to let you know that little variety is just fine!