My placement is 50/50 but my high school band director, who was a brass player (tuba, French horn and trumpet), taught me to spread my lips way out to the very edges of the mouthpiece to get a very open sound. Basically I have a gap large enough to put my finger between my lips while playing. He also taught me to open up the back of my throat ("the golf ball method"). I'm usually complemented on my sound but this lip positioning does limit my range, I can't play much above high C.
I'm having a hard time expressing this to my two nieces who are young trumpet players, one 9th grade, the other 6th.
I know I've played with some guys that use a different embouchure for high notes than low notes. They "reset" their embouchure, sometimes while playing a running musical line, based on where they are playing on the staff. An annoying habit.