The basic sock pattern that I learned way back when told me to knit the foot until it was about 2" shorter than the length [toe to heel] of the foot.
So if you're 7.5, measure your foot's length and subtract 2". Grab a friend with a larger foot and measure her foot, subtract 2".
And, since this will be such a larger foot length for a ladies' sock pattern, I'd make it 1.5" to subtract, instead of 2". This'll allow for the needed sideways stretch that will likely be needed for this larger-than-usual foot.
This is how I'd approach it, anyway.
PS - Many times, you'll find that [especially for men's sizes] the size of shoe corresponds loosely with the length of the foot in inches. But this isn't carved-in-stone.
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