Aggregate Free space for deduplication

How much free space is required to be available in an aggregate to enable dedup. IHAC that has enabled dedup on some of their volumes and the aggregate is at 99% capacity.My customer has three 4TB volumes in an 12.5 TB aggregate with dedup enabled on some of the volumes. In my customers situation dedup is only enabled on two of the three volumes which are currently empty.

It's my understanding that 4% free is required in the aggregate to enable dedup.

What is the exact amount of free space required in the aggregate for dedup?

Will my customer run into any issues in the future since they enabled dedup on volumes that are in an aggregate that at 99% capacity

All help is greatly appreciated