Hi
This is a good question, and the fact that you aware and following your stats is excellent.
Although this really is a trial and error method, using a comparison model is always good - meaning see what 'competitors' in similar fields and with similar amount of followers are charging (send them a message from a different account and ask). You can always use this information to also show potential advertisers that you charge reasonable rates - even slightly cheaper than your competition despite getting better rates.
The second method is to keep raising your price, until you see that the conversion rate (people willing to pay) start saying "no thanks" - then you'll know you've hit the max you can ask for. I personally think that 3:1 is too high (too many ads), but it would depend on the industry (not sure what exactly "news" means), and if you don't see a drop in interactions and reach, then maybe your followers don't mind.
Good luck
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