You said Flex when you spoke of a 50% reduction. Flex being Airbus's equivalent of Boeings assumed temperature reduced thrust. The max reduction is 25% as far as I am aware.
A de rate is different to flex and some operators use it to get around VMCG problems. You can add flex to your derate in which case, granted, you may well get down to something like 60% of full rated thrust. However not all operators have derates available to them or if they do use them. I believe they're used to balance up engines (we could have a mix of 20K and 22k engines a while ago so you de rated the 22k down to 20K to match the other one), address a VMCG problem or restrict a very light powerfull aircraft to normalish performance in a busy ATC environment. I think thats correct but others will have much more experience of de rates than me