Avoid Turbulance in Digital shower mixing valve

[Hienekinuk]

I intend installing a digital shower. The mixing valves would be mounted in the loft. Water feed- Hot 22.mm copper pipe Cold 15mm. copper pipe.

I understand that turbulence can be a problem if the mixer valve is close to my last elbow. What length of a straight run would I need to avoid turbulence. My water pressure is 5 bar. Flow 15l./min. Combi boiler
Any advice gratefully accepted. Thank you.