The IRS guidelines say the spread between grant price and fair market valuation. If there's a secondary market, that's your fair market, not 409A (which is a joke anyway).
Also, most companies use the last public valuation as a basis for 409A valuation rather than hiring someone to do it in a separate process. The investors buying shares are the experts here. Of course there are considerations for preferred vs common stock and things like warrants, but they start at the top line number from the last round.