(b) Maintain such fiduciary funds on California business at all times in a trustee bank account or depository in California separate from any other account or depository, in an amount at least equal to the premiums and return premiums, net of commissions, received by him and unpaid to the persons entitled thereto or, at their direction or pursuant to written contract, for the account of such persons. As used in this section, “trustee bank account or depository” includes but is not limited to a checking account, demand account, or savings account, each of which shall be designated as a trust account. However, such person may commingle with such fiduciary funds in such account or depository such additional funds as he may deem prudent for the purpose of advancing premiums, establishing reserves for the paying of return commissions or for such contingencies as may arise in his business of receiving and transmitting premium or return premium funds, or