EV = x * $3 + (1-x)[(4/46) * $7 - (42/46) * $2]


While this formula isn't high level math I couldn't possibly do this calculation during play.
I see the problem as lose $2 or win $5, or 40:100 without any outs.
Considering we have 4 outs, the rule of 4 or 2 (this is only one street so 2 in our case) gives us an improvement of 8%. 40% - 8% is 32%.
I could easily do this while playing the hand. Is this method valid or am I oversimplifying somewhere?