The best way for engineers to get into modeling is to understand it from the data up rather than from the model down. Engineers don't really need to model out the future and they tend to be bad at that. Instead, they just need to know what their important metrics are.

Even if there are less than 10 paying users in your app, you still have enough data to create the foundation:

1. Define "new user acquisition" with several categories as a funnel towards paid users. "Signed up -> has project -> is active monthly -> paid" is an example. Define the qualifications that can be programmed in to bucket a user into the category. Write code to pull this data out into high charts and see your current state of users.

2. Churn is usually harder to get the right data unless you have your events stored in log format, so approximating churn by looking at the deltas each week or month is fine to start. Create a database to snapshot your weekly or monthly metrics so you can know the deltas. Put it in your roadmap to get the data cleanly later.

3. Over time, capture the average lifetime churn as a percentage, and past quarter churn as a percentage to start, in addition to % change in users for each category over time. These are your growth rates / churn rates. You can then open gsheets and do simple modeling (no need for a Saas template yet) via a simple google search for how to start.

4. Pick the number of users that you want to have in 3 years. Take your gsheets and extend it to 3 years, and then freely move around with your growth and churn rates until that number is hit. You want to get a sense of high churn and low churn scenarios (what kind of growth rate you need to have).

5. For each scenario, how far is it from your current numbers? What will you do to move the growth rate and churn rate towards the numbers you ideally want?

You'll probably get a lot from doing the above then buying a template unless you're well beyond this stage.