I do not feel the solution set matches the Challenge definition. The solution appears to use the delta from Pi instead of the defined estimate differential delta.
The correct answers are:
9 3.1415917732 delta=4.16658e-7
6 3.1415266183 delta=3.2042972e-5
12 3.1415926414 delta=5.693e-9

Appears the expected solution can be achieved if use T(n+1)-Tn instead of the problem definition of t(n+1)-tn. The scale factor makes a significant difference.

The error noted above has been corrected.