^^^ agreed. And, your LTHR will be different depending on your activity. So, if you had actually gone to a lab to get your MHR, and even if it was done on a pedal ergometer, and if you could really calculate your LTHR from that MHR, it would be different from your LTHR on your bike on a trainer. Or your bike on the road on a TT. You just have to resign yourself that your LTHR is just an estimate, that it's exercise modality-specific, and that it will change over time, even from day to day. That's not to say that LTHR training is worthless- far from it, but its variability is one reason why it needs to be estimated periodically, and why the zones are ranges, not single values.