Okay. that must have been a glitch then. Or maybe the coupling is correct for mainstream indexes like DJI and SP and not for Dutch EOE/FTA options that I use mainly. I'll inspect again later. Thanks for replying. Ursa..
((0.5*(ATMCall+ATMPut))/ATMStrike) * 1/(0.4*strt(T)) Since, for any expiry, the second tern is a constant (for the Jun expiry, for example, it is approcimately 10), you can do this in your head. It is a rough estimate, and will come in a little high. Especially for American style options. This approximation has a name, but I can't remember it offhand. I have used it as a starting point for Newton Raphson.