"The difference between the most recent FMV (409A) valuation and your exercise price."
This will almost never be the case. This doesn't account for different share classes, liquidation preferences, preferred stock, all of which get exercised before common shares.
A better description would be "the most recent 409A valuation, minus preferred treatment, and your exercise price."
All of that is moot though, as an employee wouldn't have access to the cap table or liquidation stack. The short answer is you'll have no idea how much your equity is worth until you get the wire transfer into your bank account.
Equity as an incentive truly favors the employer. With vesting, equity rarely works out to be better than having a market rate salary, unless the company becomes a household name.