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but is there somewhere a calculator who can tell me how high the earnings could be?

I couldn't find any calculator on Minnowbooster, but @smartsteem basically provides a calculator to let you know how much you will earn from a certain amount of delegation. You can check out yourself by visiting https://smartsteem.com/ and select calculate revenue under delegation, but you must read this blockquote first, which is from Smartsteem website.

These values are not absolute and are changing based on STEEM & SBD-Price, available Reward-Fund and other variables.

I hope this might help you.

thank you for the explanation !