This paragraph (d)(6) applies to any issue of governmental bonds, any issue of private activity bonds issued to finance a facility that is required by section 142 to be owned by a governmental unit, and any portion of an issue that is not treated as consisting of private activity bonds under section 141(b)(9). Investment proceeds of the issue (other than investment proceeds held in a refunding escrow) are treated as allocated to expenditures for a governmental purpose when the amounts are deposited in a commingled fund with substantial tax or other revenues from governmental operations of the issuer and the amounts are reasonably expected to be spent for governmental purposes within 6 months from the date of the commingling. In establishing these reasonable expectations, an issuer may use any reasonable accounting assumption and is not bound by the PROCEEDS-SPENT-LAST assumption generally required for working capital expenditures under paragraph (d)(3) of this section.