What does it mean to tune down half a step?
Half step down tuning is an alternative guitar tuning. It is based on the standard guitar tuning, except that all of the strings are lowered by one half‐step. The result looks like this: Eb Ab Db Gb Bb Eb (or D# G# C# F# A# D#), that's why it's also called Eb (E flat) tuning or D# (D sharp) tuning.
What frequency is 1/2 Step Down?
One half-step down (G# or Ab) is 440/1.0595 = 415.3 Hz. (Just like an octave, a half-step interval is a ratio obtained through division, not a difference obtained through subtraction.