When i stand halfway between two speakers, with one on my left and one on my right, a musical note from the speakers gives me constructive interference. how far to my left should i move to obtain destructive interference?

Destructive interference occurs when the difference is π, 3π, 5π ..., whereas constructive interference occurs when the phase difference between the waves is a multiple of 2π.

If the difference between the phases is intermediate between these two extremes, then the magnitude of the displacement of the summed waves lies between the minimum and maximum values.

One wavelength is equal to 2π (360°) and 1/2 wavelength is equal to π (180°).

so Half a Wavelength.