Beam Dimension: 10 mm x 4 mm x 0.3 mm
Expected force: 0,3-0.4 N
The stiffness is already fixed by the geometry and material.
Thanks MarkT for your interest !
Right, simplifying by assuming the force acts at the edge only, the cantilever beam equation says the deflection will be.
2nd moment of area of your "beam", I, assuming 10mm is the length, is 9e-15 m^4 (1/12 x W x T*3)
Youngs modulus of steel is E = 2.1e11 Pa
Cantilever deflection = F L^3 / (3 E I) = 0.4 x 0.01^3 / (3 * 2.1e11 * 9e-15) = 70µm
Which suggests enough bending for a strain-gauge to work with. Thinner steel would be better of course.