There are a number of methods to find your anaerobic threshhold. I think what pro triathletes and cyclists do is actually take blood samples while exercising to determine the amount lactic acid in their system. 180 - age or doing 220 - age to get a max heart rate and then taking 65 or 70% of that to get your threshhold is obviously a very rough way to estimate. When I ride if I pay close attention to my HRM I can tell when I hit it - just above 150. Its when I start breathing hard and feeling a bit of burn. The estimating formula would say its 145 for me. Not to far off.