FITIV provides three different options for entering your MAXHR into the application. You can select between FORMULA A, FORMULA B, or you can manually enter your MaxHR if you know your MaxHR value.
MHR = 220 - age
Formula A is a very standard calculation to determine maximum heart rate, and is the formula that FITIV uses to calculate MaxHR by default. Though this formula is often accurate, individual differences in gender and fitness level can render the calculation inaccurate.
MHR = 210 - (0.5 * age)
Formula B yields a higher Maximum heart rate and can be a more accurate formula for seasoned athletes. If you find that your MaxHR for workouts performed with FITIV is often higher than the MaxHR determined by Formula A, Formula B may be a more accurate option.
There are several ways to determine your MaxHR if neither of the heart rate formulas are providing values you feel are accurate. Health and fitness testing including VO2max testing will give you the most accurate possible determination of your MaxHR, for example.
You can also adjust your maxHR according to the values you see in your highest-intensity workouts. If you find that you are never coming close to the MaxHR determined by Formula A despite putting in very significant effort it may be worth adjusting this value to reflect a few BPM higher than your highest observed heart rate in your workout history.