In my previous article on the whistle register, we explored the idea of "mixing" between whistle and head and the second passagio in operatic sopranos.
m0 = vocal fry
m1 = chest voice
m2 = head voice
m3 = whistle voice
"Unlike the usually discontinuous change (or 'break') from M1 to M2, the M2-M3 transition appears to be continuous and spread over several notes. Most singers can vary the pitch of the M1-M2 transition by several notes, and thus are able to sing in either M1 or M2 for this range, i.e. an overlap between falsetto and normal voice, or head and chest register. Some sopranos can similarly vary the pitch of the M2-M3 transition, and thus have an overlap range over which they can produce two distinctly different voice qualities (Garnier et al., 2010)."
So if the second female passagio "break" is at an E5, as we've already covered. The first study guesses a register change to whistle occurs "somewhere between C5 - E5 in the female voice. The next study I link quotes research that women can vary the position of their M3 "break." So it doesn't seem absurd to think a woman can sing in whistle on a C5 AT LEAST. However, in practical application it may be rare they might ever go below a C5 as the timbre wouldn't fit within the genre.
And this is only a soprano. A contralto, with an even lower passagio, perhaps can bring their whistle down further; how low, though? This is a very good question! I'm not sure that there's an answer to it. However, we can make an educated guess that it can go down to at least a C5.
As a light tenor I have experienced my whistle register as low as A4. However, I'm unsure that if my voice were objectively studied whether I would be in m2 or m3! I will make the educated guess, though... that a tenor voice can potentially enter whistle as low as A4.
Whether you're a baritone belter or an operatic soprano, it's important to understand your voice in it's entirety! Practice some light warm ups with an early transition into whistle voice to free up your head register =)