I'm trying to figure out how best to compare the rewards of a miles-oriented credit card to the rewards of a cash back card. I'm thinking of a simple conversion of like 1 frequent flyer mile to $0.015 . Is there a widely-accepted conversion rate?

The reason that I ask is that I've build a unique tool that calculates expected annual rewards from a person's monthly spending profile (how much spent in each category (gas, travel, restaurantes, etc.)). (You can see the tool at creditcardtuneup.com if you're interested.) I'm thinking of adding miles-oriented cards to the tool. And I'm thinking of having the conversion rate (miles to dollars) be a user-enterable parameter. If you have any other suggestions on how best to do an apples-to-apples comparison between cards, I'm open to any feedback.

Thanks,

Joel