As a general rule of thumb, I'd recommend taking the valuation of the company's last financing round and halve it. Then, multiple this valuation by your projected vested ownership percentage (and subtract any impact from exercise price).
The biggest reasons you want to discount the company's last private valuation are liquid preference and risk intolerance.
This rule of thumb is most effective if the last private valuation was recent and completed on standard terms.
Edit: I see a lot of people recommending that people value their options at $0. Imo, this is an irrational approach for most tech employees who have a non-zero tolerance to risk. Yes, options are like a lottery ticket but that doesn't make the ticket worthless, and you need to know how to properly value the ticket when a company offers you an option between shares and salary.