Emiel, I read a stack of books on cycling training from several popular coaches and learned that calculation of HR zones from max heart rate is very inaccurate. The reason for this is that because max heart rate is very difficult to determine. One has to truly push oneself to the limit. This is very difficult to do, and can be quite risky for people who are no longer young, for those who are poorly trained, or have health conditions. Instead, it is recommended to either rely exclusively on power (perhaps using heart rate as a complementary parameter), or build HR tables based on lactate threshold heart rate. Without lab equipment to determine lactate threshold, one can use FTP, and this becomes more or less your heart rate in steady state when you ride at your FTP. In any case, HR may drift quite a bit depending on ambient temperature, on how tired you are, etc., etc. The bottom line, HR zones are not that straightforward and have a limited usability, according to many coaches.