You confused a little bit pre-money and post-money valuation. If company is worth 1m, and someone invests 100k, they get 0.1/(1+0.1) (current value of company

You confused a little bit pre-money and post-money valuation. If company is worth 1m, and someone invests 100k, they get 0.1/(1+0.1) (current value of company + additional $100k after investment) worth of shares, what gives investor c. 9% of shares. Founder now has 91% of shares, what still give him $1m. (91% x $1.1 = $1m)