This has probably already been mentioned but I can't seem to find it. I decided to try my hand at btc/usd trading. I started with .1 btc just to test the waters. Initially my "trading balance" was around $150 (roughly 3:1 vs my actual btc amount). I have a much higher percentage of wins than losses. On the right hand side it currently says:

USD    2.82
BTC    0.09
Margin Balance    $46.86
Unrealized P/L    -$0.85
Unrealized Swap Cost    $0.00
Net Value    $46.02
Required Margin    $6.46

On Pair BTCUSD

Tradable Balance    $86.98
Initial Margin    30.0% (3.3:1)
Maintenance Margin    15.0%


Since my account overview says that I only have .09 in btc/usd my tradable balance should be (.09) * (3.33) * (487.46) = 145ish