I´ve heard Jim Riggio use the broken handlebar on his motorbike analogy to describe his need to compensate for the fact that his platform (TOS I beleive) underestimates the vega hit on a down move. Thus his P/L would be about half what the model predicted. I´m finding the same thing with OV. However in contrast to the P/L line, if I look at the actual OV greeks of the position and make a mental calculation they seem accurate. For example on Wednesday my position was up about $3k, and OV predicted it would be up over $4k with a drop to 2150. Delta was about -50 at 2170, and -70 at 2175 so with delta only a profit could be expected, as per the t=0 line. However the vega is -1190 to -1900 between 2170 and 2150, which would predict that for every 1 point increase in VIX the position will lose about $1k to $2k. On Thursday my P/L was at $390 which is reasonably accurate if I take the vega into account, a 2 point jump in VIX taking about $3k off profits and the delta providing about $1k. It´s as if the T=0 line is only calcultating delta and ignoring vega. I´ve attached Weds and Thus positions below. So my question is this:- is OV also a broken handlebar motorbike model? I understand it´s hard to model as it´impossible to know what change in VIX will be associated with any change in SPX (e.g., Brexit where prior to the referendum VIX spiked with little change in SPX).