Couldn't the valuation be based around both most recent private valuation, and comparatively <usefulAdjective> public companies? The bankers basically just make it up, anyway. Their goal is to just pick a number as close to what they think the market will settle on as possible. The "market" includes previous investors in the company, and also public investors of similar companies. So it makes sense the bankers would consider both markets when pricing the stock.

If you sell a car, you set the price by considering how much you paid for it and how much people are paying for similar cars. End of bad analogy.