Mineral Trust changes

My parents had established a mineral trust in 1986. At that time there was no value in minerals. So no gift tax paperwork was completed. The trust designated 50% went to them and the other 50% was split by 5 kids. in 2006 they leased out their mineral acres for 5 years. In July of 2010 they redid their trust lower their share to 2/7 and the rest of us kids recieved 1/7 which increased each kids share by 4.2%. In Sept 2010 they top lease mineral. Since the trust was change in July what gift value is associated to the kids now that the share is larger? Would it be the current mineral lease that was in affect when they redid their trust of the new top lease that happened in September? Thanks