This is a guess, since I've only been working at it for a week, but the benefit of head voice (not exactly falsetto) exercise is strengthening the muscles and nerves that produce higher pitch.
Girly voices start around middle C, 260 Hz-ish, which is within easy chest voice for most people who've been through male puberty. The problem (aside from resonance issues) is pitch inflection, which will skip back and forth over the break everyone has at 350 Hz.
I'm tackling this as a male singing problem, though I can't say anything definitive yet if that works or not. Theory goes you strengthen head voice and bridge it to chest, so you have a smooth, continuous range.
Like learningtolive, I want an unambiguous voice. (I want ambiguity points to spend elsewhere, if that makes sense.

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Actually, my model is Ashleigh Ball's character voice for Rainbow Dash. High, squeaky, boyishly sandy, but obviously female. Perfect. Tricky. Dream big, right?