* RC low-pass: phase = -45 deg (= -pi/4 rad) exactly at the corner.
* NOTE ngspice vp() returns RADIANS, so the target is -pi/4, not -45.
V1 in 0 AC 1
R1 in out 1k
C1 out 0 100n
.ac dec 50 10 1meg
.control
  run
  meas ac f45 WHEN vp(out)=-0.7853982
.endc
.end