Follow 1846's rule. For Major, company floats when concession is redeemed thus turning into a presidential share (before phase 5). The rest of the stocks goes into the company's treasury. All dividends paid goes into the treasure for the amount of unsold share in treasure. E.g. 2 shares held by 1 player while 8 shares in treasury. 100 dividend paid will put 80 into the company treasury.
A small exception to that, a company which is floated after the first 6 train has been purchased receives 100% capitalization as soon as 50% has been purchased and the remaining 5 shares get tossed into the bank pool.