Certainly. Your math is correct. As you know, you are placing parts of the total percentage number on smaller numbers. Therefore, the result is less than taking the total percentage and multiplying it by total (larger) number.
The dual rate is the only way to arrive at true cost or breakeven for the job. From breakeven, we add the profit margin we believe we can get or at least gain entry into a negotiation.
Many contractors have fat or contingency on each part of the total number. This is good in a steady economy but, can be a business mistake in a poor economy. Said another way, if a negotiator knows his breakeven or “walk away number” he can play the poker game with confidence what ever the economic conditions.
Further discussion is in the book (as you know).
Hope all is very well.