All these CBB, CBB+1 and Grace Period phase was more confusing than helping. With the release of Windows 10 1709 a lot of things were made easier. CBB is now named Semi-Annual Channel. And there is no more Grace period, no more calculating, no more dependence on any version release. You will get a fix full support time frame of 18 months.
Windows 10 will be released 2 times a year with a target frame of March and September each. After release each Windows 10 Version will be supported 18 months fix and end of support date will be available on release date or short after.
A lot of enterprise customers requested already longer support time-lines. At the time of writing this book the time was still 18 months. Look out for announcements regarding a longer opt-in time frame after release of 1709.