You don't have to go to Changsha, you can take the exam in Zhuzhou. The grading of Putonghua is a matter for the Putonghua proficiency assessment team. Just take the exam. As far as I know, the evaluation team will give the results within one week if it is generally below level 1 (such as level 2, level 2, level 3 and level 3). But if the score reaches 92, that is, a B, the marking group will send you the recording of the exam at that time (Hunan Provincial General Evaluation Center or Beijing, I forgot.

Anyway, it is the higher authorities and departments of the general examination that conduct the examination. After listening to your test recording, the evaluation team of the superior department thinks that it can meet the standard and recognizes your general test level.

PS: I got a B in Mandarin, so I will answer you based on my own personal experience. I hope my answer is what you want ~O(∩_∩)O~)