Ask HN: Computing expected value of equity

I've been offered a position at a startup with 0.90% equity. They have already raised $2MM (I'm not sure at what valuation -- should I find this out? Will they tell me?) and will likely raise more in short term.

I'm new to the whole equity thing. Let say I think the company has a reasonable shot at a $50MM valuation what would be my expected take home from an exit at that sum? (how does the dilution work?) How should I be thinking about the equity? Honestly, even if its worth zero the salary is fair enough, but I'm just trying to understand the limitations of the upside given that I think the company is likely worth less than $100MM in my guesstimate.