That it may have worked out for you was coincidence. It's one of those old wives tales. I have one hen who mostly throws cockerels. Her eggs are very, very rounded. Sometimes I cant tell which is the aircell end without candling. If that were true, hatcheries wouldn't set so many boys' eggs then have to cull the extras so gruesomely.
I have been hatching orpington eggs since March. I have had several hatches that were heavy for pullets. None of the eggs looked the same shape. i had one hatch that was 100% and 100% pullets. It had rounded eggs and torpedo shaped eggs.
The sex is determined by the hen and her XY DNA. You can't tell the sex of an embryo in a sealed egg by any other means that opening the egg and taking a DNA sample.
It is very similar to those people on ebay selling eggs they gaurenttee are fertile because they candled them. That has NOTHING to do with a fertile egg. They have been mislead and are fooling people into thinking the chalaeza is male sperm and therefore all of the eggs are fertile.
If sexing an embryo were that easy do you really think hatcheries would pay good money to Japanese men to look up little chicks vents to vent sex them? Do you think they would also hatch majority male chicks to have to cull them in greusomely cruel fashion?