I do not know how to calculate the periods for group relief when the parent and its sub accounting periods are not coterminous.
take this question for example
Sofa Ltd owns 100% of the ordinary share capital of both Settee Ltd and Futon Ltd. For the year ended 31 March 2018, Sofa Ltd had a trading loss of £200,000.
For the year ended 30 June 2017, Settee Ltd had taxable total profits of £240,000, and for the year ended 30 June 2018 will have taxable total profits of £90,000.
Futon Ltd commenced trading on 1 January 2018, and for the three-month period ended 31 March 2018 had taxable total profits of £60,000.