Mutation Frequency vs. Depth of Coverage

[AmpliSeqMan]

Tumors are composed of malignant cells, stromal cells and immune cells. Accordingly, a tumor with a heterozygous mutation may only have 40% cancer cellularity (60% non-cancer cells) and therefore (20% mutant alleles). Does anyone know the reference that determines the correct depth of coverage needed to be highly confident (>99%) that the mutation will be detected? Thanks!