From the rules:
The first certificate sold from each company must be the “4 Shares” certificate and means the player is buying 40% of that company. Once this certificate is sold, players may buy 10% shares one at a time. A company is “floated” as soon as 50% of its stock is owned by players.
I'm no 18XX expert, but this follows the method I've seen in other rules I've read. The first certificate purchased must be the President's one (usually 2 shares or 20%--in 1800's case 4 shares or 40%). Whoever is purchasing this certificate is who sets the PAR price. This first purchase does not float the company. However, the player who holds the majority of shares is the president.