[2021] Path of Exile - Petrified Blood Mechanics Explained

Your version is I think technically equivalent in any case where there are no other modifiers converting mana to something else - as you note, it breaks down in any case where the total % of max mana being converted would exceed 100%.

I think it's simpler to understand without trying to split it in two like that, but if you want to come up with a set of other modifiers that would be somewhat equivalent, I would instead suggest describing it as being like converting 80% of max mana to armour, and having a "100% more" modifier that only applies to the armour from that conversion, and not any other armour, which is pretty close to what it actually does in game - conversions are implemented in two places (calculation of thing being converted from, and thing being converted to), and this one is identical to any other conversion in how it affects max mana, but when calculating the effect of armour it multiplies how much base armour you get from the conversion by 2.