Anyone with a HRM! Esp the one from Lidl

Hi all - I bought the HRM from Lidl last week and am trying to figure out how to use it effectively. I can't find the instructions (my daughter was using it as her shopping list whilst playing shops last night and now it's gone) So - I have it reading my HR (currently 72) and there is a percentage of 38% - is that how hard it is working right now? ie I guess when I execise it will go higher. Also - should I have it set to HR or calories burned when I am working out? Any other tips on how to get the best out of it?
Thanks