A company today is worth however many billions it will be worth if it succeeds, discounted by the risk that it won't succeed. That's an incredibly difficult calculation to make, but fortunately you don't have to do it if the company recently had a funding round, just use the value the other investors decided on.

So if the company raised money at $10MM and you're being offered 0.5% equity, then treat it as a $50,000 bonus that you get for being a loyal employee over the vesting period.