for most accounts, you should extrapolate by using the same percentage of sales borne out by the preceding years' experience. you might use an average of the three historical years. you might want to use only the most recent year,or if you notice a significant upward or downward trend in an account, try growing or shinking the percentage in the future years, according to your judgment. whatever assumptions you decide upon, you should again isolate them at the top of your worksheet. so you can easily change an assumption and then have it flow through your worksheet. additionally. this is very important for calculating sensitivities later, as you want to be able to point to one cell as the column input cell in a data table.