Spread Trading and Margin Requirements

I find the whole process of entering spreads less than intuitive, and it seems to lead to trades that many brokers will not treat as spreads. Some brokers (like Options Express) don’t even consider the balancing effect for three-or-four legged trades if they are entered simultaneously.

Am I correct in assuming that I should be legging into spreads by buying my longs first? Otherwise, it seems like I could get an immediate margin call to cover my shorts.

How do I figure the margin requirements for a reverse diagonal calendar spread?

For example, let’s say I want to buy 100 close-to-the money puts expiring a month from now, then sell 100 far out-of-the-money LEAPs expiring 18 months from now. As I understand SPAN rules, I am always covered as long as I have long puts above the short LEAPS. I can’t spend the extra money that I got for selling the LEAPS, but I won’t trigger any margin or liquidity requirements as long as I cover the shorts before my longs expire.

First, is that correct?

Second, it appears that not all exchanges and brokers follow SPAN accounting rules. For example, the rules seem to be different if I am trading OEX options. Is that true? If so, how would the liquidity requiement be calcuated for such a spread on the OEX?

Thanks in advance for your help.