Social insurance for the pandemic

This simple idea from Greg Mankiw makes a lot of sense to me. What should X and N be? 2000 dollars for 4 months could be a good place to start. Total cost to the government this year = 2000*4*350 million = 2.8 trillion. Next year people must pay their grant back in surtax at the ratio of their 2020 to 2019 income. If unemployment gets really bad and incomes drop by 25%, the government will have spent (borrowed) a net of 700 billion. A quite reasonable sum as part of the necessary recovery package.