Yes Fortinagll is the Great Godfather (Godmother?) of all yews - its age must be tremendous. If 36 feet gives an age of 6,000 years then what incredible age does 56 feet indicate?
I think the Fortingall yew is now generally accepted to be 5000 years old, but I don't know who arrived at that date or how the dating method compares to the one you've mentioned