Whenever you have a margin call, offset the instrument that is generating the margin call. Don't meet a margin call with cash.
CTAs with margin to equity ratios of 12-15% are considered aggressive in today's day and age.
Margin is set by the exchanges, but can be made more stringent by the IB or FCM.
Margin levels are set to protect investors as well as the integrity of the exchange mechanism. Margin is considered a good faith deposit on the full notional value of the contract.