There are numerals on the Trim Indicator - 0  2  4  6  8  10  12  14 - and when you setup the aircraft, in CDU, on the TAKEOFF REF page, you are given trim information, i.e. TRIM 3.45, so you need to set the trim as near to that as you can (see attached image). If you have set the trim at the maximum of the green bar, 7 (half way between 6 and 8), then your trim is probably set to too much nose up, which could cause a stall after takeoff.

The only times I can remember hearing the warning horn, and seeing the config warning, is when I have either not set the flaps to my TAKEOFF REF settings, or I have not trimmed the aircraft. I normally do both of these things just after pushback/engine start, but sometimes I get distracted and will miss doing at least one of them, sometimes both. :-[

Note. I tend to set the trim a bit higher than the CDU recommended setting. In the case of the recommended trim being 3.45, I would normally set it at 4.

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