Q. What is the ruling for a woman who has a menses that occurs twice a month?

A. Scientific data does acknowledge that some woman have a situation that calls for two cycles in a month.

Your calculation method would be simple: remember, 15 days mentioned here is considered the least amount of days between ones menses, and known as point of purity.

Please refer to this article for further clarification, and other questions in this category for more knowledge.

Allah certainly knows best.