I came across this tool when doing some research on physical activity. It is endorsed by HealthLinkBC and the heartwise program. Note the caveat on medications; so for example if you are taking a beta blocker it is more difficult to achieve the target heart rate, so you need to consult your healthcare provider on an appropriate target.
Here is the link if you want to try it out. It does factor how active you are into the calculation. As always consult your health care team before starting an exercise program.