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You can't and the real way of figuring it out is a session in the rape lab however if you do high z2 and can sustain it without fatiguing a lot you're on the right track imo because carb consumption ramps up quickly past lt1 so you'll run out of steam rather fast. But focused lt1 training is for advanced athletes anyways, I still do all z2 at the high end regardless because it's just more fun especially outdoors and there's no drawback since I don't ride that many hours and I reckon that goes for all of us.
Just fyi last winter I did some testing out of curiosity where I did 2h at 70% another ride 2h at 76% and another ride 2h at 82% and there was basically no difference between 70% and 76% but a big difference between 76% and 82%, riding over lt1 eats at you so there's no mistaking it. Seiler was right