Although perhaps controversial, I have found the ability to include the distance from the ground to the first tier of branches in the TRV calculations useful in certain specific cases. Set the "Distance to First Branches" slider to 0 for normal TRV calculations. For blocks of trees that have lost lower limbs due to deer damage or, as young trees, have had the first tier stripped off due to snow and ice crust settling, set the distance to first branches to a height that will allow full spray coverage of the lowest branches.