When this livery was introduced, CIE had a bewildering collection of carriages of many origins, ancient and modern. While six-wheelers wouldn't last much longer, and no passenger carrying ones would ever bear black'n'tan, Bredins, laminates and Park Royals mingled with wooden bodied carriages of a multiplicity of designs. Most MGWR types were gone, and all DSER and WLWR ones, but among the larger GSWR stock there were many vehicles still in service, of both of the GSW's basic side profile designs. Add to this the CIE and GNR railcars, inherited GNR stock both wooden and steel-sided, and it was noted that with the plain green livery, uniformity was guaranteed - so - with the new livery it also ought to be.
Thus, exactly, as you have done above, the policy from day 1 was to have the white line and tan ("tan lines"?) edging all of exactly the same height from rail level.
This showed up as a bit odd on some older vehicles, where the division between black and tan, and the white line, were carried across the vehicle cutting panels in half, where a slightly different alignment would have looked more logical on that vehicle taken in isolation. The idea was to make visual uniformity out of chaos!