Spectra line power by medium and heavy weight elements is a main source of radiative loss in tokamak plasma and sets operational limits on the design of ITER and DEMO. The preferred comprehensive basis of impurity line power coefficients provided by ADAS is theoretical, using the Autostructure code with distorted wave electron impact cross-sections replacing earlier use of the Cowan code with plane-wave Born cross-sections. Autostructure uses orbital scaling parameters in radial wavefunction generation, which, at default settings, yield uncertainies of up to 30% in the line power coefficients. A set of orbital scaling parameters is presented for every ion up to the Pd-like iso-electronic sequence using a systematic optimisation scheme. These optimised scaling parameters are used with semi-relativistic, distorted wave Autostructure calculations and reduce the uncertainty in the line power coefficients due to the atomic structure inaccuracy to ≤ 5%. The new line power coefficients differ from the previous Cowan baseline coefficients by up to 15%. Remaining uncertainty in the composite cooling function is attributed to the fractional ion abundance inaccuracy.