The optimum concentration varies, depending on the cell type. However, a concentration of 0.1-1.0 µg/mL is generally sufficient for live/dead FACS sorting. Titrate DAPI to determine the concentration best suited for the cell type.
The following paper with protocols that may be useful for live/dead FACS sorting:
https://onlinelibrary.wiley.com/doi/10.1002/eji.201646632