Constructive and Destructive Interference: When?

[justadream]

I have gathered (correct me if I'm wrong) that:

Constructive Interference occurs when waves differ by a multiple of a full wavelength (or have the same wavelength).

Destructive interference occurs when waves differ by half a multiple of a wavelength.


But what about in between? Is there a clear way to think through this?

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[DrknoSDN]

Destructive interference occurs when waves differ by half a multiple of a wavelength.

This is complete destructive interference.
There are varying levels of constructive and destructive interference, as well as complete constructive (in phase) or complete destructive (180 deg out of phase).

But what about in between? Is there a clear way to think through this?

image from (http://www.acs.psu.edu/drussell/demos/superposition/superposition.html) Constructive Destructive Interference section

This image is showing wave addition that where the middle wave is either in phase leading to constructive interference (bold line not drawn to scale), or completely opposite to the phase of the top wave leading to a flat bold wave (destructive interference).
The bold line would have a different amplitude scale than the top 2 but the image show qualitatively that even when waves are not perfectly constructive there is still partial overlap and partial constructive/destructive interference.