Diminishing returns

On the now closed WotC forums the accepted way was to calculate with diminishing returns.
The first target is calculated with full damage, the second with half, the next with third, and so on.
The reason for this is that two Ogres at half hit points do twice as much damage against you as one dead and one at full health. A second target might not even be in the ecounter, be out of reach, etc.
Most of the times it is better to concentrate your damage.

So 15 damage to one creature is about equivalent to doing 10 damage to two creatures each.

Exceções

You do not want to waste damage, a wolf is just as dead from 20 damage, as from a 100.

On the levels where you have access to Finger of Death (average damage 61,5) most enemies you meet can easily take it, and a Fireball needs 5 targets to break even¹. However, if only 3 bugbears attack you, not 5, Fireball is still the better option.

Also, sometimes your goal is to do alguns damage to as many enemies as possible, for example to force Concentration saves. In these cases, focused fire is not your friend.

These situations are rare however.

Distrtibutable damage is the best possibility

Ataque Extra² lets you concentrate on one foe if necessary, or target another one after the first one is down.

1) ignoring spell slot levels
2) or Scorching Ray, or Eldritch Blast