Modelling a 1-2 year market downturn?

Hi. Given what is currently happening, I'm wondering what the easiest way to model a market downturn/crash might be? For argument, I am hoping to retire a year from now, but I worry that my investments will have fallen 20%, but then (hopefully) begin a recovery from there. I would rather make the adjustment to the account values, given that I will be withdrawing from them using different strategies, rather than treat it like an expense. That is, I would like my TFSA to be impacted, even though I am withdrawing from my RRSP initially. How best to do this? thanks!