The application of GAAP is not a case of mechanically applying a set of rules to particular facts. The exercise will necessarily involve accounting judgement. In the case of the GAAP standards for goodwill and intangible assets, that judgement will most often be needed in determining over what period an intangible asset in the balance sheet should be written off. In many situations it will not be possible to say that only one set of figures conforms with GAAP. For example, it may well be that the amortisation of an asset over any period between 5 and 10 years can be justified on the particular facts.
However, when looking at the range of possible judgements which generate figures conforming with UK GAAP the company must have regard to the requirement under FRS18 to select accounting policies and estimation techniques that are most appropriate to its particular circumstances. See CIRD30140 on the need to involve Revenue accountants in enquiries.