In Article I, section 2, clause 3 of The Constitution of the United States: it directly gives a mathematical method for caculating the number of districts each State would so receive. A district is meant to be as close to 30 thousand citizens without going under it; so affixed every 10 years. After the States population is divided by 30 thousand you get the expected number of seats by the whole number given in the qoutient as discarding the remainder. When America started it was 2.7 million people and 65 districts. When the Capitol Building was built the nation held 14 million people: giving 435 seats. It now has 300 million and expects 10 thousand seats.
Do not forget to account for all others to be represented: as those visiting, those in prison: or any person not taxed(illegal).