Hi Mike,
After typing in the qty to increase, you need to tab out of (or otherwise click elsehwere) the quantity field. Then the Extended Cost will be calculated based on the Average Cost of the item. If there is no qty/cost on the item (such as for a new item), the calculated extended cost will be 0.00 as you have indicated. At the current time, the process does not attempt to fallback to the Most Recent or Standard Cost in order to perform this calculation.
For now, I'd suggest putting the Most Recent Cost field as a new column on the Item grid, so that when you select the item you can see the Most Recent Cost. Then use the F5 key on the extended cost field to load the calculator and do the Qty * MRC calculation for you.
How many items do you have to do this for, just this one or many? If multiple items, you can use the Physical Inventory process to add the qty and cost (which does use the Most Recent Cost when creating the adjustments to inventory). This process is likely overkill for a change to a single item.
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Regards,
Softrak Tech Support