Ask HN: How is valuation calculated when exercising non-qualified stock options

I was granted some non-qualified stock options in a startup. These options are now fully vested and I want to exercise them. My understanding is I would own tax based on the difference between the strike price and the share price based on the current valuation. However, I don't know how the valuation is determined. I could think of a few scenarios:

* The valuation is the same as last year 409A valuation * The company needs to do a new valuation round, even though it's less than 12 months since they did last 409A

Because the company is doing well this year, I expect the difference between these 2 valuations is meaningful. Therefore, I want to know which of them (or if something else) will be used to determine how much tax I owe.

Is there a common practice for this? If I need to talk to an attorney, can I talk to a CPA or do I need to find a specific type of attorneys?