I think `prof0` should be the profile at the middle of the bandpass, not the blue limit.  The ideal choice would be the lambda-weighted average profile, which isn't very practical.  But at least the middle wavelength would be closer.  

This mostly matters for things like dcr where the position is changing with wavelength.  I think we want the nominal position (typically specified using offset) to be the value at the middle of range of offsets, rather than the position at one edge of the wavelength range.