Is there an alternative way of calculating profit, aside from subtracting costs to date from the sum stipulated in the contract?

Yes, a claim for loss of profit could also be based upon the average profit earned by the company over the past three years as taken from company accounts. This method has received the support of the courts in Harmon CFEM Facades (UK) v The House of Commons where a sub contractor lost the opportunity to undertake a cladding contract and was awarded the profit he might have earned.