Typically, small bootstrapped SaaS companies don't really know their "true" lifetime value of their customers, because they haven't been in business very long. Typically, it's calculated as LTV = ARPU / User Churn.
Based on our experience at Planio, you'll see the following patterns:
Your churn rate at 10 customers will be wildly volatile - one month it's zero and another month it is 20%.
Even at 100 customers, one person canceling in a month is 1%, so you can still see big fluctuations from month to month.
Once you get to above 1,000 customer, the volatility reduces to small changes.
Therefore, selling a lifetime subscription based on 2x your current lifetime value before you have a very constant churn rate is probably underselling yourself. You're also entering into a potentially eternal obligation in the case of SaaS in return for a one-time payment. It's hard to see how that will work unless you actually plan on shutting down in the future!
It's probably a better idea to offer discounted annual plans, so you get extra cash upfront if you need it.