I am currently installing what will be a concealed Grohe shower. There is a handheld attachment which I am struggling to understand how this will connect - as if you’ve ever tried to read a Grohe manual you’ll understand my pain. The piping needs to turn 90degrees to come through the wall (I plan to use a wall plate elbow) similar to picture. the part I am struggling with is what is then used to come from the elbow to connect to the wall mount of the handheld Does it need to be pipe that is threaded on both ends? If so what length would you make this as until the wall is boarded tiles etc you will not know the exact required length. could it be plastic speedfit I have also attached pictures of the grohe video to demonstrate the required bit. Thanks in advance