It is accurate enough for our purposes just to take your pulse for 6 seconds and multiply by 10. A good place to feel your pulse is at the wrist turned palm up, next to the large bone on the thumb side at the end of your forearm. If you are exercising hard enough you should be able to feel your heart beating and can just count that. For most people, the aerobic target heart rate is at about the point where they can no longer sing but can still talk comfortably. Exercise seems not to work through conditioning of muscles but rather through a direct, possibly hormonal effect on pain and sleep, which explains why you don't need to exercise painful muscles for the pain in them to decrease.
Daily exercise is essential.
Patients who have been exercising regularly and then miss a day usually find that their fibromyalgia symptoms are significantly worse for the next day or two. The kind of exercise is unimportant. Just make sure to pick something that doesn't make you hurt worse. It may take trying several different kinds before finding one or more types that agree with you. Jogging, vigorous aerobic dance, and weight lifting tend not to very good choices.