Maximum Heart Rate (MHR) is typically calculated with the formula 220-age. This formula is a bit controversial because it doesn't reflect the differences in heart rate according to age. The underlying heart rate formula most often used for the Bruce is the outdated Karvonen formula (below).
A more accurate formula, offered in a study published in the journal, Medicine & Science in Sports & Exercise, is 206.9 - (0.67 x age) which can also be used to more accurately determine VO2 Max, but may produce significantly different results.
A diagnostician (e.g., personal trainer, doctor, athletic trainer, nurse, medical professional, dietitian, etc.) may be best served to conduct the test twice using both parameters and formulas.