Yup, unless your textbook perfect then no method can replace actual, "what I saw on my watch", or tested numbers. It was not too far off for me, and I even had my MHR tested a few years ago. And then after a year or so, when I took up some running, my MHR hit a few beats higher yet (most likely due to te fact that even with testing, you may not hit your max in all cases, repetitive time trials and testing would be best). So I took the new max I saw occur a couple of times and re-programmed my ranges based on that. But, Karvonen is a good start for someone with no idea what their ranges are or where they should begin.

The best places for me to see a MHR are 1.) on my mtn bike on a particularly nasty climb or 2.) running (which I do not do so well at the moment).