Polar CS600: given power ---> how to adjust setting values?

I've installed a Polar Power Sensor on my bike.

Yesterday I did a test ride on a uphill and I discovered (comparing with an online watt calculator) that power value are higher of 20 watt (300 watt on Polar, 280 watt on Analytic cycling power calculator).

Can I adjust and in which manner the values of chain weight and length in order to have next ride right power values?

Yes. If you physically weigh the chain and enter in the correct value, and then take a good measurement of the chainstay length and enter in the correct value, you will then have a more accurate measurement of your power than you'd be able to get from any online power calculator with all the uncertainties associated with the myriad of assumptions needed for it.

In other words...set the Polar up correctly and believe the numbers. There's WAY too much room for error with the result of an online power calculator to even consider using that as a "gold standard".

Yes I agree with you, but the bike isn't mine. I had no time to weight the chain while installing it. Today I want to suggest a realistic value to the owner.

More chain weight = more power? or reverse?
Is the relation weight / power linear?

What makes you think that the estimate from the online power calculator is any more "realistic" than how you've decided to enter chain weight values?

IMO, either take the chain off and weigh it, or just leave it how you have it set up. No matter what you do (besides getting the correct chain weight) it will just be a guess.