So if you wanted to move 50BTC to bitcoinica how much USD and BTC should I be holding in my account at 5:1 margin?
Bitcoinica is based on USD values, so any BTC margin is effectively extra leverage. If your entire margin is BTC, and you go full long with your entire tradable balance, you'll be at 6:1. This means you can absorb drop in price by 1/6th (less in reality due to Zhoutong not willing to cut it so close). If the price is $6, you'd be sweating pretty hard at $5.25. See the threads about calculating your liquidation price.