Force comes from the design stress for the level you need — 100 MPa at a million cycles, 138 at a hundred thousand, 172 at ten thousand, 250 at a hundred — applied to the working cross-section. Motion comes from the matching strain, 1%, 2.5%, 5% and 7%, converted by the form: straight in tension, torsion of the wire in a helical spring, bending of the section in a torsion spring or a Belleville, shear of the wall in a torque tube. Heating time assumes all the electrical energy lands in the metal, which is optimistic; cooling is a lumped free-convection estimate and is the figure most worth testing. Every value is deliberately conservative, the same as the handbook. Ask us when your case sits near a limit.