The Part of Fortune was one of Ptolemy's favourite points and he believed that it was just as crucial as the ascendant and sun sign. The meaning of "fortune," however, should be thought of the same way as in the word "fortunate." It tells you what sort of activities in life will tend to be harmonious (compatible) with you. It doesn't mean that you should avoid everything else, just that a particular area is going to come more easily. In fact, the more meaningful things in life are those that don't come easily. The Part of Fortune should thus be seen as a sort of foundation. It's twin, the Part of Spirit, tells of a disposition of approach. The Part of Spirit describes how driven you tend to be naturally (without use of will). Please take note that it is ridiculously common for these two parts to be switched or just altogethered miscalculated.
Refering to the Persian uses of these parts, it was widely accepted that day births and night births were antithetical. In effect, they switched calculations depending on whether or not the Sun was up at the time. Here are the formulae as I know them:
Part of Fortune (day) = Asc + Moon - Sun
Part of Fortune (night) = Asc + Sun - Moon
Part of Spirit (day) = Asc + Sun - Moon
Part of Spirit (night) = Asc + Moon - Sun
You can see how this can get confusing if you don't already know exactly what you're doing. The even bigger problem is that these are derived numbers which depend on the ascendant. The ascendant, like the Earth, turns 1 degree every ~4 minutes. This means that if your birth time is only a little off, then you will see rapid, complex drift of the parts.
I know there are many who will disagree with me on this, but I personally believe that the point which needs to be measured is the time that the spirit reaches the body. That is the actual start of the life clock and varies between the physical birth time and up to 4.5 hours beforehand. This is the reason that I say every chart must be rectified. Even if there were someone with a pad and pencil watching the clock, waiting on that first breath, you still wouldn't have the right time. Every chart must be rectified if you want functional horoscopes. Otherwise, you're just producing some approximate information. Don't ever trust the house positions unless you have rectified the chart!