(c)in any other case, determined as follows— (X-Y)where,—X = the amount of tax calculated on the under-reported income as increased by the total income determined under section 270(1)(a) or total income assessed, reassessed or recomputed in a preceding order as if it were the total income; andY = the amount of tax calculated on the total income determined under section 270(1)(a) or total income assessed, reassessed or recomputed in a preceding order.