Apparently, I can do these, but what's my margin if I am, say, short 1 call @ 65 exp 5/18 and long the following month @ 65 exp June 15?
You can preview the order: https://www.interactivebrokers.com/...tymonitoring/checkmargin.htm?Highlight=margin
If you set it up simultaneously, there is no margin requirement, besides paying for the option spread itself. If you do the far month option first (long option), there will be no margin requirement, besides paying for the long option. If you do the short option first, it will be treated as a naked option, and significant margin will be required to start until you put the long option in place. Your broker will give you the formula for this if you ask. It is usually a significant fraction of the underlying stock or index. There are adjustments for the distance out of the money if that applies. The broker and the option exchanges quite simply do this to protect themselves from loss.