Quote from Cache Landing:
No. The mid on the SPX is always higher than the equivalent mid on XSP. But (expecially the last 4 weeks before expiry) the SPX mids jump around a lot. So when doing a r/r analysis it becomes very difficult to determine what you might get filled at. For example yesterday I had a spread limit order that was 0.25 under the mid at one point because the bid/ask was moving all over the place but still no fill.
So when I am doing a r/r analysis, rather than using the SPX mid, I will use the XSP mid (multiplied by 10) which I could almost definitely get filled at. Of course SPX has the advantage of better increments (XSP has 0.05 increments, equivalent to 0.50 increments on SPX) and lower commissions.
Yesterday, when I had an order that was 0.25 under the SPX mid. The XSP equivalent mid was still another 0.10 under that, which was suggesting to me that the SPX mid might be a bit high. If I based my r/r on that mid I would be sorely disappointed when I couldn't get filled until I had given up 0.35. Using the XSP mid ( X 10) which is much more stable allows me to get a feel for what kind of fill I would get immediately. If that is what my r/r is based on, then anything above that number is more than I had anticipated.
[edit] I should have said that the SPX mid is generally higher than the equivalent spread on XSP. Another example was today. I was setting up a put spread order and getting quoted that the mid was 0.40. I looked at equivalent XSP spread and was quoted the equivalent of 0.50 so I entered the SPX order at 0.55 and was filled within a few minutes. Had I entered the order at the original quote I would have given up 0.15 while during that time the underlying didn't really move at all.
Of course this couldn't be done with your style of trading because you are getting filled at 0.35 credits. At those strikes there is no spread on XSP.