Margins for Future Spreads

Can somebody explain to me how the margin for future spreads is calculated at IB ?
Yes I asked IB first, got a blabla answer: right click on the order then check margin.
I know that, I want no surprises, I want to be able to calculate it myself and
then I'll check before transmit.
example:
Sell VXJ2, buy VXK2 (april VIX future , may VIX future) total cost 2000 $
same question for a credit spread
 
But at least make him put you on hold to get an absolute answer. Not every guy who picks up the phone over there can speak competently to every product class. Mostly they all know equities. Some better at futures, others options.... Spot FX they will transfer you.

Ask 'em if your margin rate is higher if you're short the front month cause it's a VIX product.
 
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