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Thermal resistance is the reciprocal of U-value, so the additional resistance needed is 1/target minus 1/existing. Thickness is that resistance times the material's conductivity — with a 0.032 W/mK board, reaching 0.18 from 1.6 needs about 160 mm.
Insulation thickness
R needed = 1/U_target - 1/U_current; thickness = R x conductivity
U-value is the reciprocal of resistance. The first 50 mm cuts loss dramatically; going from 150 to 200 mm changes it by only a few percent.
0.038-0.044 W/mK for mineral wool, 0.022-0.028 for PIR boards, and 0.032 for phenolic. Lower conductivity means less thickness for the same result.