5 - 15g, would recommened extra time over CoT. Whilst these have their uses you have to understand if there is any problems with the plaster it is all on you. If you are new to plastering then find out what you can improve in yourself like zombie said.... much better in the long run.
Just realised that you where the one that was struggling on 20m2. Think it be best if you explained to people your whole technique. For instance you pva it twice but did you let it set properly first? If not it doesn't hold back. Did you coat, flatten then coat with fresh gear.... I never used too and was always chasing my tail.