Google hopes to loosen the wallets of would-be mobile advertisers with a new offering to help them determine the ROI of their campaigns, as Marketing Land‘s Greg Sterling reports today. The calculator is featured on Google’s “How to Go Mo” site, which aims to help businesses understand the world of mobile apps, websites and advertising, as well as how to use mobile to draw customers into the store or even just call them.

I suppose the calculator may be a helpful tool for businesses struggling to understand mobile advertising, but it takes about 30 minutes and requires marketers to upload data from AdWords and their websites. More troublesome, though, is that is asks marketers to make “some key assumptions” such as what percentage of potential customers who used their phones to get directions to the store actually made a visit. That seems impossible to track without offering some sort of goody — a mobile coupon or other promotional incentive — that can be accessed only through the phone and redeemed in the store.

Those kinds of mobile marketing techniques will be invaluable for brick-and-mortar stores in determining their mobile ROI with accuracy. And if they’re done well, they’ll eventually fuel adoption of the mobile wallet.