Actually, let me rvise this a bit.. wherever the Sun is CLOSEST to the Birth Sun would be the best. Because that is the approximation. On the same tithi as the birth tithi in the month, when the Sun is closest to the natal Sun.
This usually will be the same as the Sun Rasi and/or the Soli/Lunar month.
But for Adhika Maasa, I would use the one where the ecliptic longitude of the Sun is closest to the natal position.. wherever that falls.
