Tape measurements of the flaccid and erect penis were
obtained to the nearest 0.5 cm. by 1 examiner. Flaccid length,
circumference, depth of the prepubic fat pad and stretched
penile length (fig. 1) were measured immediately after the
patient undressed to minimize the effects of temperature.
Measurements were made of the length from the pubo-penile
skin junction to the meatus, circumference at the mid shaft
and fat pad depth by pushing the tape into the pubic bone.
Stretched flaccid length was measured from the pubo-penile
skin junction to the meatus under maximal extension of the
phallus. All patients underwent intracavernous injection of
prostaglandin El for evaluation of erectile dysfunction. After
a period of privacy and self-stimulation, penile length and
circumference were measured at full erection. If full rigidity
was not obtained a mixture of phentolamine, papaverine and
Prostaglandin El was injected to enhance erection, and measurements
were repeated. Confidentiality was maintained in
accordance with criteria of the committee on human research
at our university.

So to me, it is clear. they speak about pushing the tape “into the pubic bone”. so it means they pressed the skin at the body-penis shaft junction until they “reached” the pubic bone. If you put a thumb above a tape start you should be able to take such a measure