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In the ittf guide for match officials they show a diagram that suggests a range of what would be an acceptable deviation from vertical. They specfically note that 45 degrees is illegal. A lot depends on toss height if you judge by horizontal travel.. For this aspect Ma Long is not too bad. ZJK is worse. Michael Maze was the worst. I have only seen it called a handful of times.
It is hard to advise someone just coming back because so much is local in our sport. In some places the prevailing norms are more illegal than others in my experience. I try to roll with the flow of wherever I am playing because basically I don't care anymore. I am happy to play win or lose. If my opponents hide their serve then I do to. Of course I came from an era when that was an art form!