A method for a turbine blade assembly for a rotor groove is disclosed. A dovetail of the turbine blade includes a mounting arm and an end surface facing radially inward relative to the rotor axis. An extension insert includes a first end configured to contact the end surface of the dovetail and a second end. At least one positioning leg extends radially inward relative to the axis from the second end of the insert. The dovetail and the extension insert are configured to be positioned in the rotor groove to mount the turbine blade to the rotor. The mounting arm engages a radially inward facing rotor hook of the rotor groove and positioning leg(s) engage one of a radially outward facing surface of the rotor groove and a planar plate shim between the positioning leg(s) and the radially outward facing surface of the rotor groove.