I like IB a lot but their onerous policies on any VIX related trades has ruined it, for that reason I must switch brokers
I believe that most Futures brokers will simply pass on exchange minimum margins.
IBKR now charges absurd margins on VIX calendar spreads. According to CBOE the margins for tier 1 spreads (M1, M2) are $2200/$2000 and for tier 2...
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