Don Fishback sells a trading plan called ODDS which does the same thing: sell credit spreads. Sell an out-of-the-money Call credit spread AND sell an out-of-the-money Put credit spread.
Fishback uses the options' implied volatility as an estimate of the standard deviation "sigma". He tells you to choose strike prices that are at least 1.3 sigmas out of the money, which (if you believe in the lognormal distribution) means there is a 90% chance the Call spread will expire out of the money and you'll get to keep all the premium collected. Same on the Put side: there's a 90% chance the Put spread will expire out of the money and you'll get to keep all the premium collected.
Fishback tells you to use an options price calculator like Black Scholes, to compute the theoretical prices of both Calls and of both Puts. This lets you calculate the theoretical Fair Value of the put spread and of the call spread. Then when putting on the trades, use Limit orders to sell the spreads for a credit of Fair Value or better.