OK, you do the lactate threshold test as hard as you can, but at a consistent effort level (i.e. the highest level of effort that you can *maintain* for the 30 mins). The average HR over the last 20 mins is your lactic threshold HR.
Your max will be higher, as it is possible to go even harder. For example, a 1 minute all out effort. That will result in a much higher heart rate than your lactic threshold, and will probably be getting close to your max.