How do we estimate the amount of start-up funds?

I was wondering how exactly this could be done in the best way possible. We have analytics from the year before and we are using that as a basis for this coming year, however, we did shut down for 6 months and the scenarios along with the products are slightly different. Would we use an exponential percentage to get an estimate of how much money we would make in revenue and in turn use this to determine the number of start-up funds we need initially? I am a little bit lost in this matter, so any help we could get would be great!