Oh man, that hurts. As much as when I rejected an early position and at least 1% of a company that is now worth $1 B, because they demanded me to shut down a website I had at the time that was remotely related to their business and doing well ( I was super proud of it, we where getting some news coverage locally and even offers to sell) I would probably have joined if it was not for that demand, I think.

The most expensive thought of my life was "those fuckers will not tell me what to do".

I don't regret it, since it led me to the best part of my life after that site was eventually shut down 3 years later, but man... Sometimes late at night, alone over a drink, it comes back and bites.

I think the software industry might be a lot better to work in if we collectively thought "These fuckers will not tell me what to do" more often.

How much money you ended up with in cash ? Is it as straightforward as $18m×0.019 = $342k ?

Not necessarily, later investors may have liquidation preferences that would reduce it, potentially even to zero.