All of the data shown in the spread order entry form on the trading platform I used are based on the Mark price, i.e. halfway between Bid and Ask. My position statement is based on the Mark price. My trade activity is based on the Mark price. You get the idea.
Your spread was rejected because you offered it in pennies. In the SPX is must be in nickels. You can offer it at .25 or .30, but not .27
Really?! Well then, I learned something today. I'm surprised that an order has to be placed in nickel increments, but the prices traded do not...wait a minute, you sure 'bout dat?
The spread must be entered in nickles in the SPX and OEX and it must trade in nickles, but the prices used may be in pennies. For example you may see .26 and .51 for a spread that traded at .25.