First post here. New to kettlebells, but seasoned ultramarathoner. The Maffetone recommendations are legit. However, there is much more to "The Maffetone Method" than just 180-age. Stress reduction and proper diet are also key parts of his program. From what I see in my early looks, there is good information on both of those here in this forum.
Just be honest with your calculation using the Maffetone formula and be patient. No one ever got hurt from taking the -5 or -10 bpm discount. Accept the information the MAF test provides you, evaluate it impartially, and respond accordingly. Hint, if your MAF test is not improving, then your total stress levels are too high for where your are in your life at that time.