I think the percentage should be at least around the same as gay men.

Why?

1) Somebody said it here already - historically men had a better opportunity to live out their homosexuality AND they are better recorded in history anyway. Women were on the background most of the time so I think a lot of the homosexual activity was just secret enough not to get any attention by men (who were the ones writing history). For one, I have thought about it and I am pretty sure that I would live a heterosexual life as a closeted lesbian if I was born >150 years ago and either seize opportunities to have clandestine affairs with other women or just fantasize about it my whole life.

2) Bisexuality is more accepted in society than homosexuality which explains the higher numbers of females identifying as bisexuals. Again, I went through the defining myself as bisexual phase (even so I was closeted though) as a teenager because it made me feel less decoupled from everyone else and it felt less of a threat to my social status and just less scary. "Oh, yeah, girls are hot and I like them but I like boys too, all good". It is a powerful safety.

3) Heteronormative society pushes women into defining as bisexual because it is convenient for heterosexual men. That way they get access to lesbians and access to threesomes with their very much straight girlfriends. I think all of this makes coming to terms and realizing to be exclusively attracted to women a very, very hard process with lots of craziness in between. Not to speak that there are so many hetero girls convinced that you don't have to be very attracted to a guy, just like his character and how he treats you. I have actually heard about ones explaining that it is normal that sex isn't pleasurable, at least in the beginning etc. All my hetero female friends are very sexual so I didn't have that energy in my life but my ex for example had told me she is so happy she can't complain about sexual issues when talking with her friends who ALL do not enjoy the sex with their boyfriends... Point of the story: Many women would continue defining as bisexual even though they like only girls for real because boys are so easily available (ugh!), pushy (lesbians unite and go for the girls you want!), can be good friends and are maybe making an effort in relationships (which is the part where not-there-yet-lesbians think that because they are fond of their male friends, this means they are in love and everyone told them sex isn't so important anyway).

A personal statistic: in my school year we had 100 girls. From them I know for sure for 4 (including me) who turned out to be lesbians. That is 4% and it doesn't account for others who might have come out but I do not know or are still closeted. I come from a country traditionally considered as homophobic and am a 90s kid. So I think 5% is the minimum actually.