Pre-revenue, Pre-product... How To Build A Reasonable Financial Plan? (i Will Not Promote)

I’m a second-time founder, but first time raising. It’s for a pre-product, pre-revenue tech startup. I’m handling business dev/marketing, and my co-founder is ops/product. Neither of us have experience with FP&A or building out a financial model for investors.

We’re trying to figure out revenue projections and how much runway we need for the next 18mo (headcount, infrastructure, etc), but I keep stalling out on the same problem.. a lot of the inputs are complete unknowns. I know making assumptions is necessary, but I want them to be reasonable assumptions. I’ve heard the advice about not over-raising and being able to justify the numbers, sooo...

• Who actually helps startups with this? Fractional CFO? Consultant? Advisor?
• Or are most of you just figuring this out yourselves using templates?
• And if you bring someone in, how do you make sure they understand your vertical well enough to guide the assumptions?

Would love to hear how others handled this. Any tips or lessons?

I will not promote