Valuing Minerals For the IRS as an Estate Asset

I need help valuing estate minerals for the IRS. Our family owns many small mineral interests in several states. We have a yearly net income of about $200,000. I am looking for an approach for valuing the minerals that would not include valuing each property, because that would be prohibitive. In the last year we have had over 100 companies paying us royalties. I am looking for a solution that would satisfy the IRS and not cost us too much time and money.

I have seen posted here and elsewhere that the IRS values royalties at 3x ( or 4 X) annual royalties for estate tax purposes. Leased but non-producing properties are valued at 1x the bonus.

I also have seen that you need to hire an expert to perform an evaluation to keep the IRS happy.

Has anyone had experience in doing this?