That fix is right, and the reason it works is that it does not ask anyone to be brave.
The one thing I would guard against is this: any record that can appear in a performance conversation eventually gets written defensively. Once people learn the assumption log shows up when someone's year is being judged, the assumptions start arriving pre-hedged, and you are back where you started with better formatting.
What keeps it honest is reviewing it on the forecast cycle rather than the performance one. Same quarter the number was set, reviewed by the people who set it, with nobody's rating in the room. The moment it becomes an exhibit, it stops being a record.