Web App: Subscriber Growth Modeling Help

I am looking for guidance on building models for subscriber growth to a B2C software app. We have existing channels of distribution (partners) that refer business to us and we provide an offline service today with a cd-rom based deliverable. We are moving to offering less offline service and are building a web app with a monthly subscription. Interest and demand are present however we are trying to build our models and projections to determine how many subscribers we will have at X date, including many factors (i.e. acquisition cost, attrition, etc.)

I thought I remember seeing Sean Ellis talking about how to get"traction" with subscriber growth in a viral approach but cannot find the post. Any input on how to project subscriber growth, attrition, refund rate, etc would be appreciated. We do not expect to use a lot of marketing dollars on advertising but rather use existing channels to fuel growth.
Thanks,
@gesco

I honestly believe you cannot predict the rate at which you'll grow.

If you look at my blog's RSS subscriber count, for example, you'll see bumps and bends. I didn't do anything differently at those times -- it happened on it own.

What you can do is model your revenue and expenses with a given number of subscribers and determine where you're breaking even and so forth. But when you achieve a given number, it's too chaotic to know.