I would say a fixed number based on level cap is fair.
Just to make it clear, you are not saying that this is how this works at the moment, right? (Do you remember if the real buff has always been 180 or was it actually updated from 160 to 180 when the level cap got increased?)
If it was like this (fixed value automatically updated upon a change of level cap) I would be fine with it. The value we have now is not so impressive (for a "legendary" I would expect better), but it is kind of fine. It just should not stay "180" forever! The boost from crit zodiac has to increase/scale, or else this zodiac will slowly become less and less relevant..:
* perc would probably be noticeable for all players to the same extend;
* "fixed value based on current level cap" would be more noticeable for low-level/worse-gear toons;
(what "would be better and not too OP" depending ofc on the actual % value or the actual fixed value).