Thanks to all who have shown interest in this thread.
This is some of the methodology that I use in trading this spread.
I have seen it done by dividing one price by another and using that ratio (and other variations), and of course, there are other equally valid methods being used. I prefer to do it this way, just so that I use whole numbers and can calculate quickly in my head if necessary.
Multiply the S&P price by 9.5 and subtract the DJIA price (front month futures on both). This gives me a whole number price for the spread, for instance I may be "selling the spread" at 30 and buying it at 22. Each point is worth $5 per contract.
9.5 is roughly DJIA divided by S&P, and is used as the multiplier for that reason (and to get our "hypothetical mean", that is, the value the price should revert to - around 0). DJIA is roughly 10 times the value of S&P, and since the point value of S&P = $50 and DJIA $5 contract = $5, the ratio of contracts is 1:1 (S&P emini to DJIA $5 multiplier), so "selling the spread" refers to selling 1 S&P and buying 1 DJIA. The opposite for buying the spread.