Depending on the model being printed and your layer thickness I usually have to increase the top/bottom to 0.8 - 1.00mm.
That said, if the bed temperature is a little high or the hotend temperature 230C or above then this issue is more frequent. If your filament allows it then try printing at 10C less and see if that makes a difference.
going to bed but quick answer is in this guide - search for "pillowing":
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