Naked Call Margin Question

I am trying to figure out the required margin for this position:
Short naked SPX May 1120 Call @2.50
IB home page explains that is should be
100% * option market value + maximum (((20% * underlying market value) - out of the money amount), 10% * underlying market value, $250 * number of contracts). 20% above is 15% for broad based index options.

Using the margin reqs page from IB I figured that the margin should be 2.50*(15%*1095-25)=$347. It seems to be pretty low... What am I missing?

 

Looks like I made a mistake
It should be 2.50*100+(15%*1095-25)=$389
Still too low. Please enlight

 

Try this:

((2.50*100)+(1095-25))*100)*.15 = 19,800

 

Thx HelloDollars, too bad it only gives y0u 1% of return on your money in best case scenario (if call expires worthless)

 

An unfortunate consequence of a low VIX.