You have assumed that your average monthly sales over the last four months of the year will be the same as the average monthly sales over the first eight months. If there is a reasonable expectation that this may be the case, you should have a very good estimate. On the other hand, if your sales are substantially seasonal, you will not get a particular good estimate.

Example 1: U.S. Toy Sales - Is it very likely that your average sales in November and December are WAY MORE than the other 10 months. Using the average of the beginning eight months should UNDERestimate the annual total.

Example 2: U.S. Tax Preparer - Is it very likely that your average sales in March and April are WAY MORE than the other 10 months. Using the average of the beginning eight months should OVERestimate the annual total.