Would you please explaining what the logic behind 550 is? Is there any kind of difficulty level relating to 51% attack or any other stuff like that?
Basically, pruned nodes should be able to withstand chain reorganizations lasting at least 2 days. Take a look at the following quote for more detailed explanation.
The minimum allowed is 550MB. Note that this is in addition to whatever is required for the block index and UTXO databases. The minimum was chosen so that Bitcoin Core will be able to maintain at least 288 blocks on disk (two days worth of blocks at 10 minutes per block). In rare instances it is possible that the amount of space used will exceed the pruning target in order to keep the required last 288 blocks on disk.