  Understanding the trajectory of a coronal mass ejection (CME), including any
deflection from a radial path, and the orientation of its magnetic field is
essential for space weather predictions. Kay et al. (2015b) developed a model,
Forecasting a CME's Altered Trajectory (ForeCAT), of CME deflections and
rotation due to magnetic forces, not including the effects of reconnection.
ForeCAT is able to reproduce the deflection of observed CMEs (Kay et al.
2015a). The deflecting CMEs tend to show a rapid increase of their angular
momentum close to the Sun, followed by little to no increase at farther
distances. Here we quantify the distance at which the CME deflection is
"determined," which we define as the distance after which the background solar
wind has negligible influence on the total deflection. We consider a wide range
in CME masses and radial speeds and determine that the deflection and rotation
of these CMEs can be well-described by assuming they propagate with constant
angular momentum beyond 10 Rs. The assumption of constant angular momentum
beyond 10 Rs yields underestimates of the total deflection at 1 AU of only 1%
to 5% and underestimates of the rotation of 10%. Since the deflection from
magnetic forces is determined by 10 Rs, non-magnetic forces must be responsible
for any observed interplanetary deflections or rotations where the CME has
increasing angular momentum.
