@JobAndKnock There is a maximum difference of 30mm.
You are also right in thinking the 'off levelness' isn't consistent, this is what made me consider plastic packing shims.
I don't really want to use a levelling compound, I feel like its not a great long term solution. Must be a real pain for...
Thank you for the suggestions.
Unfortunately I had sound proofing put in. I wasn't aware how uneven the joists were until after this was done.
I really don't want to pull it all out as it was pretty expensive.
Similar to @KenGMac mentioned, I have seen videos of people making long strips of wood...
Ultimately I am aiming for engineered parquet flooring, however my existing floor has a slight bowl shape and the corners to corner is far from level. About 30mm over 4.5 meters.
Its an 1860 build so I think its no surprise nothing is square and level!
But I am looking for advice as to the best...