Can I use acreage instead of square feet to determine the ratio of FMV of the total compared to the sold acreage in partial sale? What is the FMV of the retained land?

59 acre forest, 47 acres sold and 12 retained.  How do I ascertain the FMV of the retained land?  The retained lot is the best part of the total acreage and could be valued higher than much of the other due to its buildablity and access to a stream.