Startups rarely die because the idea was bad. They die because the bank account hit zero mid-raise. Worse, a shrinking runway destroys your negotiating position: investors can smell desperation, and terms get uglier exactly when you can least afford it.
The rule of thumb: hold roughly twelve months of runway at current burn on the day you open your campaign. If you burn $100k a month and closing realistically takes six months, walk in with $1.2M in the bank, not $600k. A cushion is not caution.