Force corrections and space-varying smoothing-length for dust-gas. We show the dependence of all quantities for different values of the gas-to-dust scale-height ratio (η). For thin (η ≫ 1) and thick (η = 1) dust layers, the SVSL tends towards the one of the gas and of the planet, respectively. At short distances the dust-gas SLFC is restricted to the interval . The accuracy of the SL method for the gravitational interaction of the gas disc with the embedded dust is better than 2% for the whole separation range. Top: SVSL. Middle: SGFC. Bottom: relative error between SGFC and SLFC (in %): 100 · |SLFC/SGFC − 1|.