Fully agree with you here, HR and breathing govern the rest periods to a certain extent, but "feeling ready" includes a component that is often overlooked, and that is power generation... if you don't feel like the next set will be as crisp as the first, or if you feel a drop of power then don't do the next set, regardless of breathing rhythm...
Remember the swings in S&S are first and foremost a power exercise, if your power is compromised then don't do the next set! It even says that in the book
same applies to the TGU, if you don't feel ready to execute a rep in strong and perfect form then don't do it until you are certain it will be good
someone somewhere said "make the last set your best for the day" in another context, but I feel it absolutely applies here!