What is that 30% number based on? Surely that's a model specific limit, and is based more on the absolute token length, not percentage, right? I'm not sure it makes sense for e.g. Opus 0.2M and Opus 1M to both degrade at 30% of their respective context lengths.
https://www.producttalk.org/context-rot this article summarises several papers that have explored this, and it does seem to be related to both the absolute number of tokens and the % of the window