Good for you for wanting to manage your inventory wisely before you even start your online store. It's a good thing to keep in mind. Unfortunately, there really is no way to predict how much your online store will sell and your off-line sales aren't really a good gauge either. The best way to handle this kind of scenario is to:

1) Examine your competition. Try to figure out their revenue or units sold and then base your figures on whether you have more or less brand recognition than them--or if your company will be doing more marketing.

2) Instead of making projections, set goals. Set a goal of selling X amount of your product and use that as your barometer when planning. If you meet your goal, GREAT--order more product.

3) Manage your supply chain effectively. Develop good relations with your vendors and suppliers so that if you need an emergency shipment, they can get your products to you quickly and avoid a drain on your inventory.

Follow these tips and you'll stay in good shape. After the first quarter, you'll be better able to predict what your inventory usage and sales will be so that you can order effectively. And, if you ever order, you can always have a sale as a last resort. It's not ideal, but it works in a pinch.

Good luck!