I have a question on Eruvin 14a bottom which says that Shlomo's pool was round, had diameter of 10 amot and thickness 1 tefach, and circumference 30 amot (we learn from this that halachically
However, next we read (Artscroll 14a4 right column): "but there is still the little bit of thickness..." -- "the circumference is calculated from within". This seems to imply that amoraim knew that
pi>3 and thus they said that diameter of 10 amot was measured outside and the circumference of 30 amot was measured inside.
30 amot = 2 * pi * inner radius = 2 * pi * (5 amot - 1 tefach)
1 tefach ~ 0.225 amot
1 amah ~ 4.44 tefachim
and not 6 tefachim.
What is my mistake?