Rayban

In planning some excavation, I trying to figure out how far away I must stay from a nearby tree. Most of the prescriptions I've found are based on the diameter of the trunk. For example, measure the trunk at chest height, multiply by 12, and use the resulting number as the radius of a root protection zone. The problem is that the tree under consideration has two conjoined trunks (sometimes called "inosculated" trunks, apparently.) Each is about 20" in diameter. For the purposes of calculating a protection radius, what is the effective diameter of this tree's trunk? Would it be valid to combine the sectional area of the two trunks, and then calculate the the diameter of a single trunk that would result in the same sectional area? In the case of my tree, two 20" trunks would be the equivalent of one 28" trunk. Thank you for your consideration. Ray