The most important number to focus on isn't from the event itself, but in your back end: 100%. That's the percentage of your website conversions that should be successfully sent to your analytics and ad platforms. Until you get close to that, any attempt at tracking offline events will be built on a shaky foundation.
Brad Redding explains on Honest Ecommerce that the core issue with most analytics is incomplete data. When you rely only on browser-side tracking, you might lose 10-30% of conversion data to ad blockers or iOS privacy features. His solution is implementing server-side tracking, which creates a direct, reliable connection between your store's backend and platforms like Meta or Google. This ensures they receive 100% of conversions, each packed with the maximum amount of attribution data possible so the platforms can accurately match sales to the ads that drove them.
Once you have that solid digital baseline, you can start connecting your offline activities. The key here is to shift your goal for the pop-up event. As Kunle Campbell described on the 2X eCommerce Podcast, you should prioritize "secondary conversions" over primary sales. In this context, a secondary conversion is capturing high-quality customer data, specifically an email address. You can do this with a simple tablet or QR code at the event, offering a small discount or a gift in exchange for their email.
Now you can bridge the gap. You take that list of emails from your event and upload it to your ad platforms as an offline conversion set. Because you've already been feeding the platform perfect data through server-side tracking, it has a rich history of user activity. It can now cross-reference your uploaded list and identify people who attended your event and had also previously seen or clicked on one of your digital campaigns. This won't capture everyone, but it gives you a far more accurate picture of your campaign's influence than you could get otherwise.