If you can determine what the FMV of the entire property was on the date you inherited it, you could allocate a % of that value to the easement, using total square footage of the property, and square footage of the easement.
A real estate appraiser may be needed, unless you have an Assessment Statement for the property.
This is probably the best way to do this, not having a value for the easement itself.
Here's some additional info from the IRS on the subject:
https://www.irs.gov/publications/p551#en_US_201812_publink1000256931
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