AImrie
Torque calculation uses the friction coefficent under the bolt head and in the thread. They are not always the same. Therefore, it will be better to go to my opinion with the equivalent K value. Where: Torque = K*Bolt_diameter*Force.
If you measure the bolt load and the torque you can calculate K.
If you assume the same coefficient of friction under the head and in the thraed then you can calculate it from K.