What about choosing appropriate concentration gradient of your cytotoxic stimulant? No formazan crystals means no active mitochondrial dehydrogenase enzyme.
For further reading on colorimetric assay methods to assess survival or proliferation of the cells please refer to:
Vega-Avila E, Pugsley MK. An overview of colorimetric assay methods used to assess survival or proliferation of mammalian cells. Proc West Pharmacol Soc 2011; 54: 10-4.