The point of those numerical relationships was to point out that if there are two daily windows that maximally allow 30' runs then it is pointless to be concerned with what could be possible with a single 60' window and so forth. If there is time enough to go 60 then why would anyone opt to go 40? If there is time enough to go 60 at one point and 30 at another point then why would anyone not do both (as opposed to 40 & 30 or 60 & 0) if they have adequately built their training volume to that point? Everyone who wants to do their best will work towards maximizing the volume within the windows of free time they have available. It is doubtful that anyone who would be interested in running doubles would also be interested in training below what they and their schedule could handle.