How strict on HR for base training?

Monitoring heart rate is great when you're running easy pace recovery runs. It helps you ensure you're keeping the stress on your body low. Just account for HR drift and it's great for recovery and foundation runs. Stress on your body is impacted by all of the variables you've described, so again, HR can be a useful metric for easy days.

HR can also be helpful during longer races where you need it monitor things early on to avoid late race crashes.

However, HR is not as useful for specific workouts where you're focusing on certain aspects of training. The general consensus is perceived exertion, pace and power are all more useful metrics for workout days.

Forget about base building and go higher in heartrate , 70 -75 % MHR when running easy runs!! Go with me the magic wizard and make wonders 2022!!!

Run by feel and leave the HRM at home. waste of time.

MAF is not scientific and there isn’t a single scientific peer-reviewed paper validating Maffetone’s claims over nearly four decades (beyond obvious ones like always running very slow is less likely to injure you, all else being equal), and Maffetone himself is neither a medical doctor nor a PhD.

Low-HR or equivalently easy-effort base building has a long history (Karvonen, Lydiard, others) but there is no value, and likely disvalue, in being religious about arbitrary HR limits.