I would always recommend to sell the idea first. If you can get customers to commit to pay "3 months in advance money back if we don't deliver the product 3 months from now", you are solving a real problem and won't have an issue bootstrapping development and growth through customers.
Otherwise I would also do a back of the envelope financial model. We published DjaoDjin's financial model for boutique SaaS a couple weeks ago (http://djaodjin.com/blog/breaking-the-curse-of-negative-cashflow-in-saas.blog.html) but you can find other models as well. Google "SaaS financial models". Plug in some numbers in any model that seems appropriate for your business will give you an idea of when you break even and how much runaway you need to get there.