It's not really reasonable, because what one investor or syndicate thought the company was worth at one moment in time is neither what the company is worth now

It's not really reasonable, because what one investor or syndicate thought the company was worth at one moment in time is neither what the company is worth now nor what anyone else thinks it's worth. And it's certainly not what it will be worth if and when you are able or forced to liquidate your shares. If you're trying to assess the value of the equity you're being offered, the last round's terms are perhaps worth knowing, but they're not the result you want. What you need to figure out is not what someone else thought the equity + terms was worth to them, but what the equity + terms you are being offered is worth to you. This applies to all situations in which you're being offered anything other than cash, whether it's a contract gig with a startup or a dude on Craigslist offering to trade you a stereo system for your laptop.