Well, there's no magic formula. I was read as male a lot when I was in my thirties and back at the university after dropping out. Younger crowd, lots of young men, and I was young-looking. The older I got, the less I "passed." But when I was at my unhappiest and quite overweight (much of it in my chest), I still confused people sometimes, even when people didn't know my (male) name. Go figure.
I started T at 46 and was just starting to be read as male FAIRLY consistently right before top surgery. I had top surgery 4 1/2 months after starting T and never really had issues after that. I may have been openly misread a couple of times the rest of that year--occasionally, a male cashier or service representative would say either "ma'am" or "man," and I couldn't tell which. I feel that top surgery was a big game-changer for me, on top of the rapid changes from a full dose of T.
I suppose I'm lucky in a lot of ways. I already had a masculine presentation, I didn't have a pretty face, and I have never smiled much (women tend to smile a lot more than men do). I'm not unusually short for a guy (about 5' 7"), my shoulders are not super-narrow, and my hips are not super-wide.
I feel that some of "passing" comes from the company that you have been keeping. Virtually all of my socializing was with men, and I have always been in relationships with men, consistently from high school. I figure that I picked up a lot of mannerisms and habits of speech that are more common among men than among women. I have noticed in my own trans community that the trans men who have been in relationships with men before transition tend to come across as straight when they have transitioned, and the ones who came out of the lesbian world tend to come across as gay, mainly because they have a lot more stereotypically female mannerisms and seem femmier. I don't know whether the guys of lesbian provenance take longer to be read as male, though.
Mixed bag. See what top surgery does for you.
P.S. I'm also blessed with really aggressive sideburns, and my facial hair started becoming very full after a few years in. I am unfortunately losing my hair in front, but all of these features add to the overall picture. Facial hair is awesome if you can grow it--once it matures and doesn't look like teenage facial hair.