Using HRM for weight classes kettlebell bodypump ect?

Hi,, hoping someone can answer me this,, ok so I use a HRM which is a good quality one and is pretty much on track with the machines at gym and other calorie apps it has all the features one been calories burned now I do kettle bells which is a 30min class I put all my effort in to it and usually only burn 230cals using. 4kg kettle bell as I have got strong and my fitness has improved no today I upped it to a 6kg weigh sweat was dripping off me and I really pushed myself and my results after class were 290clas! My friend told me to keep the monitor on and keep track of calories burned because with these kind of classes and bodypump your body's is still burning SO I have kept the monitor on for another 40mins and it's now showing I have burned 377 calories!!! All I have done is drive home and sit down pretty much so my two questions

How do I record my calorie burn for this session as the calories are still burning?
When do I take the monitor off??

Is this correct am I doing something wrong lol

Thanks in advance