The Retail Owners Institute is a fantastic site that any retail owner (including optical stores) can use to manage for FREE their offices. The self help site includes retail calculators, financial management basics, training courses, inventory, profits, cash, store issues, store benchmarks and more. This is a membership program for all the information and or fee based webinairs.
That said this is what they said Inventory Turnover is for 2010.
The reason that you want to calculate inventory turnover because it quite frankly is how you make money. The faster the turn, the better profitability. A high inventory ratio means the product is selling. You should also calculate inventory turnovers per category (i.e. accessories, kids, sunglasses, readers, contact lens etc.)
Inventory Turnover Ratio = cost of goods sold / average inventory.