The way I used for the double lunar month (the leap month):
Before 15th of the leap month –the same month.
After 15th of the leap month — the next month.
For example: Dec 20,1984 was Oct 28, 1984 in Chinese calender. Because it’s leap month and “28” is after 15th, so we will use Nov 28,1984 in Chinese calender for zwds chart. If not 28th, for example, 10th of leap month, it’s before 15th, so we just use Oct 10th in Chinese calender for zwds chart.
If right on 15th — then I would look at both charts to settle it down first.
Anyway it’s a good idea to settle down every chart before looking at the future.