I think the 0.003/prince over 25 from the wiki is correct. The screenshot on the wiki is from version 1.30.1 in which you got 0.004/prince over 25, so the calculation would have been (78-25)*0.004=0.212 which matches the 0.21 which is shown in the screenshot. And in the current version you get 0.15 for 78 princes at the start of the game which matches the expected value of
(78-25)*0.003=0.159 (I have no idea how you arrived at 0.17 for the same calculation):
View attachment 1126627
Do you maybe have some imperial authority growth modifiers? I think they are included in the per member calculation already and then they are counted again to calculated the final value(this might be a bug).