I also use a launcher app (keypirinha), and it has a nice set of options to determine placement in this setting. Here is an except from its config help about its options (mouse_monitor is what I like):
# * auto:
# When displayed, the window will always appear on the primary monitor,
# horizontally centered, with a width proportional to the monitor size
# * monitor_1, monitor_2, ...:
# Same as 'auto' but allows to specify which monitor to use.
# * active_monitor:
# Same as 'auto' except that window will be displayed on the same monitor
# than the currently active application.
# * mouse_monitor:
# Same as 'auto' except that window will be displayed on the same monitor
# than the mouse pointer.
# * mouse_pos:
# The window will be positioned according to the coordinates of the mouse
# pointer. If the default size of the window is too big, the window might be
# moved and/or resized according to its functional constraints.
# * persistent:
# stores window's geometry (position and size) so it can be
# restored the next time it is displayed, even after a restart.