It is calculated based upon the longitude of the Sun, Moon and the Ascendant, and therefore composed of the three most significant points of the chart (only the Midheaven isn't directly involved in this calculation). In Ancient Astrology, the Ascendant is the primary indicator of the physical body and health, while the moon is the living aspect of the body – soul. The part of fortune being composed of both of them along with the SUN, tends to indicate the link between the physical and social world in which we live.
When we consider prosperity or health , if a part of fortune is in a good placement , placed near benefic's ( such as Venus, Jupiter and Sun) it will indicate that the physical and social world will support the person and will allow him or her to live in a good condition.
Conversely if malefic's are involved (such as Saturn and Mars) the person will have a harder journey.
Malefic is not synonymous of evil, rather it implies these planets help us build our character and strength. This can only be achieved when the person is proactively working on his/her consciousness, otherwise the experience will be more complex.
Another interesting point is the system of 'derived houses" and how this system can help us understand better the Part of Fortune.
For instance, let's take a chart and spot its location – if we mark the house in which Part of Fortune is located in the chart as house number 1st , than we will count the next house as 2nd, than 3rd and so on.
That way we can see the connection between the Part of Fortune and the other areas of the person's life.
In that example, the house in which the Part of Fortune is located as number 1 and we will count from it to the 10th house , and that house will give us an additional information about the person's best career and how he will make a life and a living. This 11th house (counting from the location of the Part of Fortune ) will show us the social role that the person is playing etc. and so forth…
Conclusion – by finding Lot of Fortune in the chart we can get an important additional information on how the person can be successful in area of his career and health, how the person will enjoy life. It will all, of cause depend on the fact that the person will activate those abilities that he has.