YES how one sits is very important for MY success at being perceived as ladylike. Now, before I respond with what I feel is truly helpful info for you based on my successful life experience and some super advice I got earlier in my transition...
I posted last year about 9 months ago, some of MY successful techniques for passing well with female-model looks and behavior -- but I got strongly attacked for my successful experiences and advice SO I left this site for about 9 months -- so this time let me give a TRIGGER WARNING -- TO ANSWER YOUR ORIGINAL POST WELL REQUIRES THE FRANK DISCUSSION OF SOME GENDER STEREOTYPES AND EXPECTATIONS ABOUT WHAT PEOPLE ACCEPT AS "ACCEPTABLE" FEMALE BEHAVIOR. If such discussion causes you discomfort, please stop reading here. So I have forewarned you.
""" Trigger warning -- Okay -- here is how I was trained and what I've had fantastic success with, with being perceived as a beautiful female model and seldom can others detect I am trans. If THIS is your goal also, my experiences shared here will likely help you a lot. If this is not one of your goals, or if you feel that my advice relies too much of expectations and stereotypes, then please disregard it.
Here is my specific highly successful experience and advice:
1) You will NOT likely be perceived well as female unless you sit with your legs together or sit with your legs fully crossed. Why ? Because sitting with legs far apart, taking up lots of space, is usually perceived by most people as a strongly masculine behavior. You will likely be perceived as demure and feminine if you take up less physical space -- because women most often do exactly this -- try to take up as little physical space in public as practical -- watch most women in real life and you will see that this is correct.
2) Men usually sit with their legs far apart because they are masculine. We women -- cis (non-trans) and trans -- usually sit with our legs far apart only as an invitation for men to approach us for sex. If this is what you want at any given moment, then by all means go for it girl ! And I hope you enjoy the attention. But if you don't want men to perceive you as being approachable for sex and you look otherwise highly feminine, then don't sit with your legs spread apart. Period !
3) If you are a trans lady and don't look particularly stereotypically feminine otherwise, if you sit with your legs far apart you won't be perceived as a girl who is asking to be approached sexually. Rather, in this case you are more likely to simply be incorrectly perceived as a "man" and it will probably be much harder to "pass" as a woman.
4) I am pre-op, but medical HRT hormone treatments for years have shrunk my unwanted genitals to a rather small size -- so that I am physically able to cross my legs tightly without physical discomfort nearly to the degree of any post-op trans lady or nearly to the degree and tightness of legs together of any cis (non-trans) lady. This helps me, and it might end up helping you, too. Although, if you cannot do this, don't overly worry about it. I've known several feminine-looking trans ladies in person in my life who CANNOT cross their legs -- either due to lack of physical agility to do so and/or because it physically hurts their unwanted genitals to do so -- and still look fab fem just the same. They simply always sit with their legs closed rather than crossed. Works just as well if this is necessary in your case to accomplish this, if this is your goal.
This precise advice was given to me years ago by both cis female friends and trans female friends, and wow-oh-wow has this advice served me well especially recently !!! Good luck in being as feminine as you want to appear and being perceived as feminine to the degree this is your goal and desire and your feelings. Jennifer xx