Here are all the caveats and wrinkles I can think of:
* Ownership percentages are deceptive. If you're an employee (meaning: you start with a steady salary), 1% is a big number. With 19 employees preceding you, you're probably not getting 1%. After funding, the founders may be only high single digits themselves.
* 90% of 0 is 0. You need to be asking questions about how equity might become worth something. This isn't as hard a conversation as you may think it is. Startups often sell for rule-of-thumb multiples of revenue. Ask: two years from now, what's the top line revenue of the company going to be? Will the market value the company at 2x that revenue (a consulting company multiple), 4x (a product company), or 8x (a product in a red-hot market)? Take all the numbers you get and work back what the company is saying you might make if the company gets bought, and then scale it by the likelihood that any private company gets bought favorably. Otherwise, all the "percentage" you get is is a point score for your ego.
* (most importantly) Even if you know what the shares are worth today, you don't in fact know anything about what they're worth by the time they become liquid. That's because every single financing event in the company will reallocate ownership. Every round of funding is going to impact them. In extreme but not (unfortunately) rare cases, your shares can be worth nothing even in a tens-of-millions-of-dollars acquisition --- this goes double if you leave the company before the liquidity event. A company with a stock plan and 20 employees has spent a lot of money on legal to ensure that you have no rights.
I just got back from a huge industry convention in San Francisco, and after talking to a lot of friends there, I've come to this conclusion: tech people are unbelievably awful negotiators. Let me make a suggestion. Do what you'd do if you were buying a car. Negotiate the value of the transaction in an objective currency: dollars. Then, when the company tries to "finance" the deal in shares, you at least know the dollar value they're trying to assign to the shares.