To all.
There is NO "calculation" for MHR. It is what it is. It can be discovered by performing a ramp test, as per the web page I quoted upthread.
OP describes his activity like he is a seasoned athlete and performed a pseudo test by running at 180, and then sprinting to record his MHR. 203 was it? So now he knows. After a few weeks of intervals, he can re-test and update his charts.
If any of you have any suspicion performing a ramp test might prove fatal, go to your doctor to arrange an ECG test with the practice nurse. Your doctor will advise you if there is any abnormality in your heart function.