Enter a race that takes 45 minutes to 1 hour where you really are trying to do your best. Your average HR over that time will be a good estimate of LT. Of course the most objective way to measure it is by blood samples on a stationary bike, but I believe some experts actually define it as average effort at a race pace over that amount of time.

True max HR is difficult to actually measure and will indeed vary among sports. The calculated max numbers are worthless, don't even think of them even as "better than nothing". They aren't.

Once you peg your LT through a good old fashioned 1 hour race pace effort, you can figure out your other zones for various training goals.

By the way, an interesting phenomenon is that as you age, your LT will tend to become a higher and higher percentage of Max. Usually because you can maintain your LT or even increase it, but your max will decrease over time.