Sometimes when I login I can see a stat of what percentage my customers use Shop Pay. I can't find this anywhere now, and I would like to know what it is as I have messaging on my typical cart page that Shop Pay users will not see.
As Tobias mentions below, the best way to do this within Shopify would be to download your transactions and calculate the percentage in Excel.
You could always try our Report Toaster app. We have a Transactions by Gateway report which provides a breakdown of all payments based on the gateway used by your customers. Please feel free to take a look and let us know if you have any questions.