* Exits north of $100MM are rare, and a $400MM exit is rare indeed; virtually any such exit will be from a famous company. Valuations are at least somehow tethered to sales, and companies that justify mid- 9-figure exits can usually consider IPO... as an example of how rare that event is.
* In most sectors of the industry there are rule-of-thumb valuations based on multiples on sales. An enterprise software company aiming for a $150MM acquisition is expecting 4-8x, and needs to be achieving 18MM (optimistically) to 40MM (conservatively) sales to do that. You can reconcile this estimate by asking for current sales, this year's "number" (in a well-run company, everyone knows the number), and then asking "what's going to happen to scale the number up".
* Last time I had to think pragmatically about VC, a round that took participating preferred shares (in which the VC takes their money off the table, then takes their percentage off the table) was an indication of a weak round; if they're shooting for the moon, you're entitled to hold that against them.
* Finally, remember that if you quit, the equation changes again. When you leave, you can execute your vested options, but that costs money. Perhaps nobody in the company is less protected than former employees: investors have contractual provisions to protect their money, and employees are given retention grants, but former employees can be written right out of the deal. I've seen it happen.