Ready aim is applied the first time you fire with a weapon at a squad, and is reset whenever you change which squad you are targeting
So quick 3 examples:
M10 fires at a PIV, uses AP rounds ability (switches weapon): Ready Aim
Brit sniper kills someone from an enemy squad, continues firing at the same squad: Fire Aim
Brit sniper kills someone from an enemy squad, player commands it to attack a different squad: Ready Aim.
Thanks for the clarification. So I guess your DPS calculator is based on the the first firing cycle after acquiring a target. I should probably tweak the equation a bit to account for a prolonged combat setup. But anyway, this does not seem to matter too much for small arms after all where a unit's DPS matters most.