I just looked through AC 103-7 and I do see how you got the idea that mean chord is at mid-span.

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The term mean chord is misused in the advisory circular. Keep in mind they're using it in the context of measuring camber:

The MAC is not simply the average of all of the chord lengths. For a tapered wing, that would be at the center of the

*length* of the semispan. MAC is the

*centriod *of the semispan, and on a tapered wing that is inboard of the midpoint because more area is inboard. Dan Raymer gives a nice way of understanding MAC in

*Simplified Aircraft Design for Homebuilders:*
You really owe it to yourself to get a copy of this book. It is not aimed at ultralights

*per se *but it covers many topics and concepts you will need to understand in the course of designing your plane.

There is also a geometric way to find MAC for a given wing (google for more info).

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Now, all that being said, I don't think MAC matters (strictly speaking) for now. Here is an equation that will be helpful, expressed two different ways:

$$\text {span} = \sqrt{\text {aspect ratio x area}}$$

$$\text {aspect ratio}={{\text {span}^2} \over {\text {area}}}$$

You can find the area needed (based on the AC103-7 chart; it will be more than 100 ft^2). You can find the aspect ratio of a full-size P-51 by measuring the span & area. Now that you have the area and desired aspect ratio, you can find the resulting span.