As a Shopify store, you can use Shopify's reports to calculate your repeat sales percentage but it takes time to do it all the time, especially if you want to watch it over the calendar year.
Repeat Customer Insights now includes the Repeat Sales metric
Now Repeat Customer Insights includes the Repeat Sales percentage metric for all stores.
If you're an existing customer, just login to the app and you should see that metric on top. I'm already calculating it for you so there's nothing for you to do.
If you're not a customer of Repeat Customer Insights yet, install the app the app. Once your store has been imported, you too will be able to see the repeat sales percentage (along with a dozen other repeat customer metrics).
If you're not sure if Repeat Customer Insights is valuable enough to you, signup below and I'll teach you more about the repeat customer metrics that are included in the app.