For that particular case (buy back at -50 delta, why not place the order based on underlying >- Strike? That way you are not subject to broker producing unreliable Greeks! However, placing a market order on an option is generally not best idea as you may get trip to cleaners. I do not have IB, but TOS allows placing a limit order based on offset from MARK (say MARK + 25cents) at the time of the trigger condition, which I have used. Mileage may differ depending on liquidity of the option.