Glad you have a HRM, and it is working. My guess would be that your max heart rate is whatever your max is. I think that is how they would measure it in a lab. Get to your highest, and that is it.
I think we just had a similar diiscussion in another thread.
In any event, why not take a moment and read this article - it may be very enlightening about MHR.
http://faculty.css.edu/tboone2/asep/Robergs2.pdf
Have a great day.