I find optial stock to be useless unless you're buying things individually. Nearly everything from a wholesaler comes in cases of 6, 8, 12 etc so setting a number for optimal stock is no good. I just leave it blank.
Optimal stock field means the value of optimal stock set at the item’s form in the selected store. It need for Autofill option in purchase orders. Quantity field of added items will be automatically filled by the formula: