If you want to write off the entire purchase price then put a "one" in the first month.

FASB If you select the FASB method then the number of months of postitive cash flow are computeed and the fraction of the total amortization is determined by dividing each month by the total cash flow for this positive cash flow period.

Other methods There is a wide range of methods used to amortize portfolios. LLAMA does notr attempt to provide you with these calculations. You can create and of the non-cash flow methods directly with your spreadsheet or calculator. There are nor rules and you can set the excat amoutn for each month of the valuation.