I don’t think it’s been changed. I looked at a high sec athanor and it still seems to obey the same calculations. I am not sure from where you are reading the number. I am checking by trying to reprocess some ore and hovering the % display at the upper left which shows 54% as base for this particular athanor. A tatara should have a superior percentage with a T2 rig.
Reprocessing Rigs modify the Base Yield of the structure itself, they do not give a “bonus”. All structures only fitted with a Reprocessing Plant has a base yield of 50%, now if you put on a rig (T2 in this case) the base yield is changed from 50% -> 53% and then the role-bonus of the structure is applied so that is 53 * 1.04 (Tatara) = 55.12%.
Tatara with no rig: 50 * 1.04 = 52%
Tatara with T1 rig: 51 * 1.04 = 53.04%
My calculation formula, in the post you linked, is still very much correct.