So we've established that long 100 shares X short two calls is a short synthetic straddle. It's simply a matter of reducing the transaction to 50 shares X short one call to trade a half-sized synthetic straddle.
UCO's synthetic (forward) is trading at shares. I am long 50 shares from 52.03 and short a Mar19 51C from 4.11. There is 1.03 of intrinsic in the call, so I am short the half-lot 51 synthetic straddle from 7.19. This half-lot has the same payoff as the full size (one contract) Mar19 51 straddle. Stress it as a full lot and reduce the payoff by half.
It's the only method by which to trade a listed half straddle. The only issue is if converting the half straddle to a fly as you will be synthetically buying twice the wing exposure. Buying a one lot strangle to convert the half lot to an iron fly will result the same payoff as a one lot synthetic iron fly with two lot wings (stressed at 50% payout for the half-lot).
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