When valuing mineral rights for estate tax purposes for mineral production from horizontal wells, is largely dependent on the amount of proven producing and proven non-producing reserves developed. If your father took volume depletion over percent depletion on his tax return, it is even more important for tax purposes. However, if the acreage is small or the minerals are a small part of the estate, third party offers made to purchase at about the time death may be acceptable or a factor of the last 12 months net income using discounted cash flow projections based on the recent production records.
Don't be shortsighted when establishing mineral estate value. Talk to your siblings and make a plan for the mineral inheritance. Perhaps you would be wise to form a trust or LLC for the minerals with the option of selling long term production or the leasehold itself. In that case, the tax basis should be a high number if possible.
Also, if your father could have owned more OK minerals than the land included in the existing lease, leave the possibility of discovery open.
If the total estate is large, seek the advice of an estate accountant so you don't allow an underestimate in a small portion (minerals) cause an audit for the entire estate.
There is no one size fits all. Prioritize.