If the Sun is below the horizon, then you should use the night formula. If you want to check manually, the night formula for the PoF is Asc + Sun - Moon
. This provides the position of the Sun on the Moon chart, as adjusted to the ascendant. Note that it is adjusted to the ascendant, not 0° Aries. While sign-related information may still be usable, the house-related information is what was intended to be reliable. Also, because of this "multi-chart" nature, longitudinal aspects to parts might be nonsensical. I haven't studied that part myself yet, but I'm sure someone already knows.
P.S. Hooray for rising Virgos!