Osteotomy starts at #13 (Fig.5).  In fact, the trephine bur is off the target at the site of #14, not immediately inferior to the sinus floor downward extension (arrowheads).   It would be safer if the trephine bur were used for 1 mm and X-ray were taken.

Internal Sinus Lift Perforation

Xin Wei, DDS, PhD, MS 1st edition 05/20/2015, last revision 05/21/2015