An alternative method of accounting shall be used if the taxpayer or the administrator demonstrates that the net profits of the taxpayer allocable to the city cannot be justly and equitably determined under the separate accounting method or the business allocation percentage method, or if undue expense to the taxpayer would result from complying therewith because of the taxpayer's manner of operations and methods of accounting. In such case the administrator, upon application of the taxpayer or upon his own initiative, may approve or specify factors or methods of determination as will effect a just, nondiscriminatory and reasonable result. Application to the administrator to substitute other factors in the formula or to use a different method to allocate net profits shall be made in writing and state the specific grounds on which the substitution of factors or use of a different method is requested and the relief sought. No specific form need be followed in making the application. Once a taxpayer has filed under a substitute method, he shall continue so to file until given permission by the administrator to change.