Maintenance margin %

Newly opened IB account (in US) and trying to test out shorting a call option. Stock imlied vol is around 50%, a bit volatile but not crazy.

Seeing that maintenance margin is about 35% of full exposure of buying at the strike. They probably have a complicated formula based on tail scenarios etc, but was expecting something lower, e.g. 20%. Just wondering if this is normal margin requirement for shorting options at IB?