Easiest way for me...first figure out the UY your trip will take place...then you know that you can used banked points for the UY prior to the trip, points from the UY of the trip, and borrow from the UY after the trip. So, since an April 19 trip is in your Dec 2018 UY, it can be paid for with banked 2017, 2018 and borrowed 2019 points.
With a Dec UY...and I have one...its best to not even think about the calendar year as the majority of your UY is in the next calendar year!