Model + Stage = One Metric That Matters

By having these diagrams you know what business you are in, and what stage you are at.

Based on that you can focus on the ONE METRIC for that specific model & stage.

In a Startup - Focus is really hard to achieve, its your biggest problem.

http://bit.ly/BigLeanTable - PDF with explanation

Must have metrics

If you are not growing your customer or your revenue by 5% a week you should not be a startup!

In the early stages of growth, if you cant find 5-7% of growth a week - you need to have a long hard conversation with your founders.

Knowing the normal is important, your 3% doesn't mean anything.

Baseline:

30% users/month use web or mobile app
10% visitor engagement/day
1% will be on the site concurrently.

2-5% churn is pretty good ( the best SaaS companies get 1.5 - 3% a month)
You get to this before you move to your 'scaling' stage. 
eg: Facebook crying pic (reduces abandonment by 7% - EMOTIONAL appeal works well here)

Calculating Customer Life Time

If you know 25% of your customers leave every month, then average Lifetime of a customer is four months.

25% churn = 100/25 = 4 months
5% churn = 100/5 = 20 months
2% churn = 100/2 = 50 months

Figuring out what the acquisition cost should be?

Rule of thumb - Don't spend more than 1/3 of your customer LV on your customer acquisition cost.

Eg:
5% churn = Lifetime of 20 months
Paying $30/mo = $600 LV
1/3 spend = $200 
Now you are still lending $200/-
At this stage you segment and find out who sticks around longer - eg - twitter user are longer term users, moms etc.