[carbocation]

The point that fishcakes raises is one important element. Make sure to understand the picture fully (not just the % of common shares currently outstanding, but the % of the company if all preferred shares were fully converted to common stock, etc).

There is something of a red flag in how you've phrased your question. It's curious that you say that you will be joining the company, but then your post is all about the fact that you haven't yet negotiated the equity you will be receiving. This makes the most sense when you only are considering joining the company. If you already have an agreement to join, then this agreement most likely stipulates what equity you are owed, and on what schedule it will vest.

Part of the overall package that is required to convince you to work for this company should include a specific % of the company's equity. Surely your bargaining position would be stronger if you hadn't already agreed to work for the company when making an ask for equity.