Menstrual cycle and ovulation are two very different things.
The menstrual cycle is the regular natural change that occurs in the female reproductive system that makes pregnancy possible.
Formula to calculate menstrual cycle.
Menstrual cycle begins on the first day you have regular bleeding until the first day of your next regular bleeding.
This formula is more applicable to females with a regular menstrual cycles.
If May 5th was the first day you had regular bleeding, then June 3rd was the first day of your next bleeding, then we can say that the length of your menstrual cycle is 30 days.
However, keep in mind that your cycle may vary in length.