Antares,
Fortunately Primary Directions are about as clear as mud by most if not all of the ancient authorities. There is a very good reason for their being called 'Primary' which is obvious by its name. Today people tend to jump straight to 'Secondary' progressions and transits as a way of predicting, even though the most obvious (for specific reasons) Primary should be first. Without some 'primary' directions, secondary progressions are almost meaningless.
Firstly primary directions are/were only used from the Mc, though other reliable authorities such as Ptolemy, Antiochus, Rhetorius and Valens etc used to direct the Five primary places; Mc, Asc, Sun, Moon & Lot of Fortune. Their methods for directing were about as diverse as their names, if not more. I will summerize a few.
The most common method in use today is to take one degree in Right Ascension across the Mc as equal to one year in the life of the native. Right Ascension has to be used to maintain the correct angle between the natal ascendant and Mc.
Another method crudely takes one (longitude) degree and adds that to any planet or angle in the time method of one degree is equal to one year of life.
Yet another crude method takes the actual daily arc of the Sun on your day of birth as equal to one year of life. This also can be added to any planet or angle as above.
Other versions use the latitudes, others use zero declination by directing the ecliptic degrees and others the mean solar arc. There are other methods too.
Personally I have been using Ptolemy's method as laid out in his Tetrabiblos. This method divides longitude degrees into the proportionate times in respect of the rising times of any given sign. It is quite a complicated process and I do not know of any program or website that calculates them in this manner. Because of the complexities of the math and the human evolution of becoming lazier these more precise methods have been all but forgotten.
Hopefully this makes them a little clearer for you, lol.
Bob