The under apron comes across about to the left front of your left thigh (mid-way between front and side of thigh). The relationship to width of over apron will vary based on how wide you have chosen to make your over apron, so that 6 inch figure is not hard and fast. The under apron needs to be wide enough to provide coverage when you sit down, but not so wide that it gets in the way while walking.
You could make it wider, try it on before attaching Velcro (use safety pins), walk around/sit and see what you think. Easy enough to make it narrower at that point -- nobody will see if your waistband is less finished on that end!
Sorry it's not a precise answer but that is part of the "craft" nature of kilting! And others may well offer differing advice.
Proudly Duncan [maternal], MacDonald and MacDaniel [paternal].
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