Optimised formula constants for the Hoffer Q (pACD), the Holladay 1 (SF), Haigis (a0/a1/a2), and Castrop formula (C / H / R).

Formula constant optimisation was performed to minimise the sum of squared prediction errors PE. A) refers to the ‘classical’ formulae with standard nK/nC values, with B) the formula constants and nK/nC in the main part of the formula were varied for optimisation, with C) the formula constants and nK/nC in the main part of the formula were varied to minimise for PE and the PE trend error over corneal radius, and with D) a standard optimisation was performed using the nK/nC value from situation B) derived from the other dataset in terms of a cross-validation.