What I do is to look at how much a stock has moved over a similar time frame and try to pick strikes with a 90%-95% success rate. I limit it to historical periods when volatility is similar to today as measured by VIX. I do this mainly for SPX and AAPL trades, but just loaded some CMG data for you.
Here is how CMG looked on March 19th and what I would consider for April 17th spreads. The number of trading days holding would be 21 so I look at all past monthly op ex moves from 21 days prior. I use current VIX * 1.3 for limiting volatility to similar periods.
This spreadsheet then takes all the historical 21 day % moves and intra-period high and low % moves and converts it to today's CMG price.
A Bull Put Spread with a short strike of $575 or lower has a 92% success rate of not being pierced over the 21 day holding based on the past 50 month moves when VIX was below 19. If held until expiration, only 3 times of the 50 months did it close below $575.
I did not go back to look at what a CMG put spread with that short strike pays. I can typically get around 3-5% monthly when doing this with SPX spreads.
Here is what the CMG analysis for a 21-day trade looks like (these are trading days not calendar days).
http://screencast.com/t/ddLoDhzC
You can use some broker platforms to get the probability OTM percentages to do something similar. But that looks at probability at op ex only... I want to know how much the stock may move up / down intra-period as that is what would shake me out of a position.