We all like to THINK our first flat serves are big enough to win at least half the points, and solicit weak returns the other half, when they go in, of course.
But in order to GO for the first serves, we need a super reliable, basically hard to attack second serve, which is usually much slower, higher bouncing, and placed to the area WE decide. And we still get a tough return of half of those!
This against our peers.
Pretty much, when my first flats go in 50%, I cannot lose at my level of 4.0, unless I hurt myself and default.
Fortunately for me, 50% is not realistic in serious match play, so I'll settle for 35%.