Initial inventory placement ("first allocation") is really important. But it will never be perfect because of the dynamic nature and volatility of ecommerce. At this size, regular inventory rebalancing will be required to ensure that inventory levels are as optimized as they can be for future orders.

There are a few ways to do it, but the primary concept is to blend probabilistic purchase forecasting with current inventory levels. If there is a high probability that a certain SKU will go out of stock in the East Coast FC but many customers will likely purchase it, then it might make sense to send some of that SKU as a bulk shipment between FCs. This will prevent long-haul and split shipments in the future.

While it's always good to have optimized inventory levels, the volume likely isn't high enough until around this stage of the business to make it an economically sound project to prioritize.