When calculating strength/stiffness, is it conventional to assume that all sheet metal is at the worst case thickness given the sheet tolerances (so 0.025" aluminum is treated as 0.0225", assuming 48" sheet width)? Or is there some fudge factor to account for most of the sheet not being at the absolute worst case? Does this then get pessimistically combined with worst case material properties (295 MPa tensile, 245 MPa yield for 6061-T6, vs. 315 MPa / 280 MPa nominal)?