Three years carry back of losses

I know that there is a maximum cap of £50,000 on the amount of loss that can be carried.

If the loss making accounting period is less than 12 months, is the £50,000 limit time apportioned, to
arrive at the amount that can be carried back.

OR
Do you time apportion the profit making period down to equate with the loss making period, to arrive
at the amount against which loss can be offset.

Many thanks,