METsThe simplest way of estimating calories burned in exercise is what's known as METs which stands for metabolic equivalents. At rest a person is at 1 MET. Exercise will be some multiple of this amount.
1 MET is approximately 1 calorie per hour per kilogram of bodyweight. I weigh about 85 kilos right now, so I burn about 85 calories per hour doing nothing.
Martial arts training, like HEMA, is going to be 5-8 METs. Most of the training is going to be around 6 METs, and using that as a baseline value is a reasonable estimate. While high-intensity training or sparring can get up to 10 METs, this is typically alternated with rest periods so over the course of an hour it will average out to something lower - like 6 METs.
At 6 METs, 1 hour of HEMA would be 85x6 = 510 calories. This is consistent with my own values from my heart rate monitor.
The Compendium(pdf) is a resource that estimates METs for a very wide range of activities. This can then be used to help you estimate calories for all of your exercise and fitness endeavors.