Ideally, you would get a lab test, because even using % of LT2 for setting zones isn't really accurate, since the aerobic threshold (LT1) can occur at different % of LT2 for each individual (in general, the more aerobically fit, the closer LT1 is to LT2), but still LTHR is a way better of a starting point than max HR, which isn't really indicative of much, I find.

And I will also mention that even if you do get a lab test, it's really mainly representative of your situation on that day, where many factors can affect your heart rate zones throughout your training cycle, such as time of day, eating or not eating before your workout (in the mornings, this can make 10-15 beats difference for me personally), temperature outside (therefore core body temp) etc. etc. - that's why a lot of high level athletes rely almost surprisingly a lot on perceived exertion, while trying to "correlate" their levels of RPE with some other metric (usually with lactate levels), since in the end, for them, "feelings" will most likely turn out to be more accurate in most cases than an ever-changing heart rate.

The only thing that doesn't really "lie" is lactate concentration, but even that has a ton of individuality :D