IPO valuation calculation

Hello. Can someone help clarify how a company's valuation at IPO is determined?

X company files IPO, raises 100 million dollars and is valued at 400 million dollars.

How is the 400 million calculated from S1? Do you just multiply the IPO price with all the shares (common, preferred, warrants, options). I calculate it for one company as an example but it does not seem right.

Thanks.

Wayne