Everything is quantifiable, especially to Sherlock. If it exists, it can be measured; sometimes it's simply a matter of coming up with units.
He calls his units of caring (in his head) milliWatsons. If something bores him, he couldn't give a milliWatson. If it's transport, like food or sleep, that he needs, he might give a centiWatson (ten milliWatsons). If he comes across Irene, well, he might feel nearly a whole Watson then.
Bonus: while on the run post-Reichenbach, he's idly contemplating how much he give no shits (no milliWatsons) about something, and suddenly realized he's drastically undercalculated how much he misses John, and therefore has to revise the whole system, since he was off in his most basic unit of measurement.
Sherlock/John, in measurements of you
He calls his units of caring (in his head) milliWatsons. If something bores him, he couldn't give a milliWatson. If it's transport, like food or sleep, that he needs, he might give a centiWatson (ten milliWatsons). If he comes across Irene, well, he might feel nearly a whole Watson then.
Bonus: while on the run post-Reichenbach, he's idly contemplating how much he give no shits (no milliWatsons) about something, and suddenly realized he's drastically undercalculated how much he misses John, and therefore has to revise the whole system, since he was off in his most basic unit of measurement.