The last time there was a recession in 1991 CBA bad debts rose to 1.5%.
CBA currently have about a $900B loan book and make about $8B in NPAT.
So their threshold for bad debts to loans maxes out at 1% before they become loss making.
If there is an elongated recession (rather than just the virus coming and going) I'd also think $40 as a fair level with a P/B of 1 and Trailing PE of 8.