THanks for your question - sorry for the delay in getting back to you. THe
diagram on this page
(http://pps99.cryst.bbk.ac.uk/ppscore/section3/jonc/revturn2.html)
is certainly labelled wrong - residue i+2 should be labelled i+1 and i+3
should be labelled i+2. I shall have to check out the definition in the
literature to answer the other part of the question but it sees logical to
me that it would be the torsion angles of the centre residue - i+1 - which
would determine the type of turn.
I hope this helps.
Best wishes,
Clare Sansom
c.sansom@mail.cryst.bbk.ac.uk
Coordinating Tutor, Principles of Protein Structure