Since the basis adjustment from the sale of the permanent easement affects only the land, how you use your residence should not influence your cost basis calculations. Remember that land isn't depreciated.
If you owned the property when the previous easement was sold, the land basis should have been adjusted at that time. (You might need to verify this.) But if you purchased the property more recently, the property records at that date should have taken the earlier easement into account (because the easement no longer belonged with the property you purchased).
Your best option is to use the square footage as of the date you purchased the land (assuming the prior easement had been adjusted out). Calculate the percentage of the easement you sold and multiply that by the acquisition cost for only the land. The result is your basis adjustment and the offset for the easement sale proceeds.
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