A volume/intensity split is useful when there is a weight the trainee can do for a single set, but might fail on the second (or alternatively, might not be able to recover from more than a single heavy set). It is a similar programming rationale to topsets and backoffs.
Around five reps is chosen for the volume to keep it isochoric with the set of five, which is the most productive rep range for strength training.
If you can do more than one set at your 5rm, you are probably not suited to the Texas method. If you cannot, your 5rm is your starting point for the intensity day.
This is all in the book.
Hi Mark
I got the book, I'll have to re read it as clearly glossed over or forgot that detail 😂