What is the industry standard for mark to market, given that there are no settlement prices? I notice that IB uses Last, but in the case of the indices (and ETFs of) which trade until 4:15 pm, Last sometimes is far from the bid/ask for certain strikes as volume seems to dip and spreads widen after 4:00 pm I considered valuing at either; 1) liquidated at bid/ask, ie short leg covered at Ask, Long sold at bid, or 2) cost of setting up the trade at closing prices, ie Shorts at Bid, Longs at Ask I am not looking at the tax situation, nor am I particularly interested in the margin impact, I simply want an accurate way to value open positions from a risk management perspective. Could someone who is trading or has traded options professionally advise how the book is valued?