When do I read the No Regrets volume?

I recently found me a really good deal for the manga of AOT and bought it ASAP. When I got there, there was an extra volume (No Regrets) and I was confused as to what it was. After a searched about it I found out it was a spin off about basically Levi's past. Now I have up to the 27th volume of the manga and was confused as to when I should read the spin off. While I was looking into it I saw people saying to read after the 4th, 5th, and up to 12th volume but I was not sure and I wanted to get a concrete number before starting the manga. I'd appreciate not getting spoiled at all and thanks for the help if I get any🫰