There's more to it because "valuation" does not mean that the investors actually thought the company was worth that much post-investment. They almost certainly

There's more to it because "valuation" does not mean that the investors actually thought the company was worth that much post-investment. They almost certainly got other terms as well and those clauses probably constituted the bulk of the value they received, not the equity itself. Without understanding exactly how the deal was structured and what the terms were, you have no way of even guessing what the investors thought the equity was worth. There's also the fact that the round didn't close 10 seconds ago; things have undoubtedly changed since it did. Those changes could positively and/or negatively affect the value of the equity. Once you're past that, your approach is basically ok. It's a bit like the Drake Equation, though, in that it's several unknowns multiplied together to achieve a very precise result that has such large accumulated error bars as to be meaningless. The best you can do with this approach is try to avoid taking a worse deal than the last investors, which doesn't mean much. Also, no one has a 25% chance of success unless they've filed an S-1. But that's just a detail.