Default [Q] App assessment using a web landing page

Hi Guys,

I need a good advice on mobile app assessment.

I was suggested the use a landing page as an assessment tool prior to the development process.
The idea is to build one page website, which will describe the advantages of the mobile app.

The page will offer to enter a user's email address with the promise that the app will be sent to the provided email (because it's in a beta stage - so it's not on the market yet), now, once a user will enter his email, he will not get any email about the app.
The point of all this is to assess the need of the app by calculating the number of email submissions divided by number of visitors.

I was told that a reasonable percentage of registrations should be around 40%; less will be count a NO GO in terms of business value.

I would like to hear your opinion on those matters:

- Is it common practice to use a landing page to assess mobile app?
- What is the weight of such assessment tool?
- Is it recommended to solely use this kind of assessment tool?
- What registration percentage can give a good feel on the app demand; is it around 40%?
- What is your recommendation of assessment strategy before developing mobile applications?

Thanks ahead