Going to side step the progression level question, although I agree with the comment above.
The TR crew tends to focus on intensity, and so for low-intensity endurance base work one article that I’ll bring up is this one:
which makes the case that an average trained athlete should stay above 45% vo2 reserve, which is roughly 68% HRmax.
190 * .68 = 129bpm
So if you are trained and fall into that average then stay above 129bpm.
So thats the bottom and what’s the upper boundary for low-intensity endurance?
The article also gives an estimate for the upper bound of low-intensity, 77-81% of HRmax.
190 * .77 = 146bpm
190 * .81 = 154bpm
Assuming those averages apply to you, that puts your low-intensity endurance work at 129-150bpm where I simplified the upper end and split the difference.
Targeting the middle, that gives a target for endurance rides at 140bpm and you can then equate that to a power.
Another way to approach it is if you have some data on “all-day pace” so for example if you have done some long 8-16 hour all day rides what was your average bpm?
Personal example - I’ve come off a 5 week break and on second week am targeting around 60-64% of my pre-break FTP. A couple weeks ago I started out at 30% pre-break FTP 
Right now I’m doing shorter 1 hour endurance efforts. Today I’m going to extend time out to 2 hours, then once power/HR stabilize at 2 hours its time to push out to 3 hours, then 4 hours. Based on prior seasons, over time I expect the target power to go up to 70-75% at the same heart rate. Another interesting thing to track is aerobic decoupling.
Hope that helps.