Optimise horizontal lines at peaks and troughs of a sine wave

[babelproofreader]

I'm trying to create/optimise a Tukey Control Chart on a sine wave such that
the control limits (horizontal lines) are as close as possible to the peaks
and troughs of said sine wave. What syntax/function can I use to extract the
positions of the peaks and troughs of the sine wave from a selected portion
of its vector so that I can extract the values of the control limits at the
corresponding positions from their respective vectors?
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