Trill fingerings are so interesting: they are not 'one size fits all' as we first learned them! I find some of the basic fingerings lacking in pitch accuracy: the top note might be too flat or the tone quality too false. Occasionally, there is a magical mechanical fix, such as the C# trill key. It is one of the best investments not only for the B-C# trill but for third octave G-A trill, so I highly recommend this particular key as an option when purchasing a flute. One of my favorite sources for alternative fingerings is Nestor Herszbaum's book "Alternative Fingerings for the Flute". It has many different solutions which are helpful depending on the color/pitch you might require. One of my current finds is the third octave E-F# fingering. The typical thumb trill yields a very flat F#. Try fingering A natural with the left hand, right hand on 2nd trill key and trill with the A key in the left hand...so much better.