Tweak damage heuristic again

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Date:      Tue May 1 21:12:34 2018 -0400
#
# On branch master
# Changes to be committed:
#	new file:   ai_metrics.lua
#	deleted:    fleet_dispatch/fallback_fleets.lua
#	modified:   handle_fleets.lua
#
