When is it time to unleash the Big Gun?

Solution 1:

Most players get used to the tempo of the gun, so overcharging isn't an issue.

Most players move backward while charging so hitting the wall at end of the tunnel as in your screenshot won't get you hurt.

It shares many characteristics to rocket launcher so they have very similar tactics. You can inflict damage with indirect hit. It may not kill the enemies but from indirect hit it saves lot of ammo for other guns. Most boss moves slow so it is not difficult to have direct hit on them even at long range. My preference is to use it at medium to long range, which the enemies are trapped or behind the wall.

You may score lucky hit in multi-player at medium range, as everyone keeps moving and jumping. It is also suitable for suicide attack at close range, although it is kind of difficult to get a BFG in multi-player game and the charge up time would becomes an issue. The psychological effects at close range is also great to novice player too. You may also scare them to have mistake while jumping on difficult terrain.

You can have more information from here.