How can you create a look back audience to send an existing flow to?

I’m curious to know how to send a flow email to a past group of customers that fit the parameters of the flow, but did not receive the flow email because it was not created yet. For example: 

• Customer purchases our ring sizer, and they receive a 20% coupon to use on the purchase of their ring 
• The flow sends a reminder email to those who purchased the sizer and have not made a purchase since, after 30 days.

I guess what I'm asking is, can I send this to previous customers who qualify for the flow?