Why only a $40 credit? optionsXpress is reporting NBBO of $0.35/$0.85. In my experience, they usually fill pretty close to the middle ($60). This obviously raises the expected profit.
At a $60 credit, I see the math as
(-240)*.0908 + 60*.8122 + (-90)*.097 = -21.79 + 48.73 - 8.73 = 18.21
which is an expected profit, as opposed to an expected loss at a $40 credit. I'd calculate the return as 18.21/240 = 7.59% (or roughly 13% annualized).
(Apologies if you've already covered your rationale for defining the size of the credit. It's a long thread.
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