Sorry guys, but that's us. This is from 220 Tri and is an alternative to the "220-age" formula; it also means you can set a different MHR for each sport - I don't know about you, but I have different working HRs for cycling and running. And as cardiac fitness changes your zones can adapt.

The rule is really simple: trunble along gently breathing through your nose. Increase intensity until you just have to breathe through your mouth. That's 80% of your MHR right there.

(Caveat, this is from Joe Beer who once recommended energised water)