Originally Posted by Creakyknees

I've trained with hr since the late 80's and am familiar with the problems of the OP. Short answer is, there is no short answer - you are in the zone of uncertainty.

When I ride tt's, I just make sure I'm rested and feeling good, and in the initial miles try not to get above the threshold zone.

But if I'm feeling really good after the first few miles (and depending on my pacing strategy for the tt) I might just open it up and go hard based on perceived exertion, ignoring the hr.

Which, IMO comes down to the real way to go fast in tt's: make it hurt and ride as fast as you can.

My best tt results have come from that strategy; and during those rides my hr was several beats above my "usual" threshold zone.