Greetings Bill:
I'm not sure which formula you are using to estimate age, but the only recognized ways to establish tree age are (1) Knowing the age of the tree when established or planted, (2) Using an increment bore to "estimate" tree age, or (3) Cutting the tree down and counting the annual growth rings.
Sorry, but the Illinois Big Tree Register only recognizes the largest native tree species here in the state -- we don't have a register that recognizes the oldest trees. Such a register would be nice, but we simply do not have the resources to handle that daunting task.
Thanks for your inquiry and happy big tree hunting!