How accurate are period and ovulation calenders?

Pretty good. These methods. Including basal body temperature, as well as the calendar/ rhythm method and cervical mucus checks, have been used successfully for number of years. Far earlier than the advent of over-the-counter devices. However, the urine pregnancy test is extremely sensitive and specific for pregnancy.
Depends. If you typically have regular menstrual cycles on the predictable pattern these calendars are quite accurate. If your menstrual cycles are not regular then they're going to be very inaccurate.