Outdoor Training with no Power Meter

I am trying to figure out which Heart Rate value to use when I training outdoor since I don’t have a Power Meter. I have completed the Full Frontal last week. My new HR values

Max HR is 180
LTHR is 168

When a outdoor work out say:
Interval
Effort: 8 minutes @ RPE 6 – 80% of FTP – HR <90%
Recovery: 12 minutes @ RPE 2.5 – 55% of FTP – HR <75%

Do I use my Max HR or LTHR HR to set the heart rate for the workout.
Max HR would be 162bpm
LTHR would be 151bpm
That is a big difference on effort between the two HR.

Any help would be greatly appreciated !

There used to be a cheap heart rate monitor that converted hear rate to power. I think it was made by Powertap & was okay for longer intervals. Maybe it was called “powercal”. Personally I use a cheap power meter on my 2nd bike that doesn’t measure power directly but calculates it from altitude gain, wind speed and rolling resistance. I find it’s entirely good enough for a second bike. Currently having a mental block as regards name. Company is called “velocomp”. They periodically do discounts & the cheapest is, I think, $199. “Powerpod lite” is possibly the name.

If you look in your profile in the SUF app you can see the HR zones based on your LTHR. My Z1 and Z2 have much higher upper limits than the traditional %Max HR calculated values which I was riding to outdoors and would explain why they felt utterly couchlandrian.