1. Export current CSV from sku grid.
2. Click File --> Make a Copy to save a copy of your CSV in case you make a mistake. You will make your changes on this new copy and keep your original.
3. Go to your My Items Page and click Add Single Item (you're not adding an item, we're just going to use the formula wizard to create a formula so you won't have to read it)
4. Click Override Default Formula
5. Click Use Wizard
6. Use the formula wizard to create the formula that you want by filling in the values in the wizard.
7. Click Close on the wizard.
8. Copy the long selling price formula that the wizard created.
9. Go to the selling_formula column in the CSV and paste that formula all the way down so that all of the items will have the same override formula
10. Be sure that your reprice_sku column looks like normal ebay item numbers. If you're using Microsoft Excel it changes your numbers and you would have to click on the column header, then right click in the highlighted area... select Format --> Number --> and format as a Number with 0 decimal places. Other software such as Google Sheets or Libre Office (both are free) will not have this problem.
11. Once you're sure your CSV is good save it.
12. Go to Sku Grid and Import your revised CSV
Once the file has uploaded, it may take some time for everything to be updated in the system.
13. Then force reprice all of your items at one time under Batch Options --> Force Reprice All Items.