Whenever any value (firepower, movement, etc.) is affected by more than one modifier, always perform any addition and subtraction before any division or multiplication.
Important: Any unit that has its firepower halved more than once may not attack.
Halving firepower
In many situations, you must halve a unit’s firepower: If a unit is supporting an attack with combined fire, if a unit has been activated with a Fire and Movement action, if a vehicle is heavily damaged, etc.
You may never halve a unit’s firepower more than once. If two or more effects would force you to halve a unit’s firepower, the unit may not attack.
For example, a heavily damaged vehicle cannot support an attack with combined fire.
Whenever the rules instruct you to halve firepower, round up.