I think on the center vs two ear measurements it makes less difference than many think. Even over a somewhat wider area.
Here are some sweeps in REW done 1ft right and 1ft left of the center LP. 1/24th smoothing. Looks uneven especially below 1 khz.
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But it is worth remembering this isn't how it will sound. Our ears do about 1/6th octave smoothing at the highest frequencies. And then 1/5th, 1/4th and so on until it is 1/2 octave smoothed in the bass. Here is the same measures 2 ft apart at 1/6th octave smoothing.
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For the most part above 1 khz there is only a fraction of db difference. I'm not showing it because it clutters the graph, but the center measurement between these two points for the most part splits the difference in the two measurement points. So if a REQ measure was done at the LP it isn't going to be grossly wrong a foot or so either side of it. It would be even less difference 4 inches each side of the center measuring position.
Next are the same two points with Effective Rectangular Bandwidth based smoothing which mimics the smoothing our ears do. It is a little closer still and much better below 1 khz. This is all with the 500 msec IR window to preserve LF resolution.
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