Dunno, I'm not an expert in constructs with more than 2 legs.
Just this: there's no free lunch, there will always be a remaining risk.
Your approach in your OP was not bad: you have to work with probabilities, the more of them you bring into the range of 0.5 to 1.0 the better...
Since one cannot eliminate all of the risk, then it makes sense to concentrate on a specific range around the initial price: for example 1 to 2 SDs..., ie. ignore the extremes at both sides...
But this can be used only when one intends to keep the options till expiry, b/c then only the underlying price decides (as then IV no longer plays a role anymore).
Here's some code on how to calc xSD, ie. pass z=-1 for -1SD etc :
Code:
double calc_Sx(const double S, const double IV, const double DTE, const double rPct, const double qPct, const double z)
{
const double s = max(DBL_EPSILON, IV / 100.0); // must be > 0
const double t = max(DBL_EPSILON, DTE / 365.0); // must be > 0
const double r = rPct / 100.0; // risk-free rate, earnings return etc.; can be negative
const double q = qPct / 100.0; // dividend rate; can be negative (CHECK)
const double u = r - q; // drift
const double Sx_at_zSD = S * exp(u * t + s * sqrt(t) * z);
return Sx_at_zSD;
}
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