Although I don't exactly disagree with the other answers, my opinion is that the question is looking for a perfect mathematical system where there likely is none. I don't think the author of this book was intending an exact method with precise "rules" for "summing" the stress patterns.
Rather, as this is a book on songwriting, my guess is that it's just an ad hoc method for collapsing many levels of potential rhythmic/metric stress into the 2-3 levels of stress needed for text-setting. I'm assuming the author chooses three levels, with the intermediate stress level able to take either a stressed or unstressed syllable, depending on the text.
My reason for saying this is because I think before worrying about the mathematical system to create summative patterns, one first would need a rigorous mathematical way of defining where the rhythmic stress indications come from. And from the examples given, I'd say that's rather ad hoc, based on the local amount of rhythmic activity rather than the global "weight" of various beats or stresses. In the examples given in the question, one could identify 4-5 levels of stress, but that isn't helpful as a guide for text-setting. Instead, for text-setting, one needs to identify patterns of strong and weak syllables that roughly alternate. (It's rare to have more than a few strong syllables in a row or a few weak syllables all in a row.)
Unless there are more clarifying examples in the book, the summative just appears to be a rather loose way of combining both the global (metric) stresses with local (rhythmic) ones that are more dependent on the specific rhythm. Or, to state the principles in words, I think the author is basically saying, "Match stressed and unstressed syllables to the local rhythmic patterns, but don't forget to also give particular weight to the stresses created by the overall meter."
However, all of this is a tentative interpretation based on only the limited excerpt given in the question and some of the comments -- I haven't had a chance to look at the actual book for clarification.