What's the cost basis for selling direct listing RSUs?

If you had RSUs before the Slack direct listing, what cost basis do you use for capital gains / losses when you sell your shares? For traditional IPOs you use the IPO price (eg. $72 for Lyft), but I'm wondering what value you use for a direct listing. I know there was a "reference price" ($26 for Slack) before the listing, but my understanding is that was mostly a meaningless number.