As far as I understand it, it is the exchanges that actually set the overnight margin rate but have little interest in the intra-day margin rate that brokers offer.
Current exchange and IB requirement is initial margin 3938 and maintenance 3150. It is illegal to require less then that. Maintenance margin 3150 means that you need to support this level to keep a position overnight. 3150 < 3800. You need 3938 to open a new position after 15 PM in Chicago.