(e) Rents for dwelling units in projects or parts of projects constructed for elderly persons of low income shall be computed as provided in section thirty-two; provided that in the case of persons receiving old age assistance under chapter one hundred and eighteen A, directly or indirectly in whole or in part, from the commonwealth, dwelling units in projects or parts of projects constructed under section thirty-nine shall be deemed to be adequate housing for elderly persons and shall qualify for and rent at the maximum rental allowance under the old age assistance laws, regulations, and policies. Notwithstanding any provision of law to the contrary no elderly person of low income or handicapped person of low income shall be required to pay more than twenty-five per cent of his or her income without utilities or thirty percent with utilities for rent for dwelling units in projects or parts of projects constructed or leased or purchased under this chapter. For purposes of calculating the rent of elderly tenants in state-aided public housing, local housing authorities shall treat pharmacy costs reimbursed pursuant to section 16B of chapter 118E as deductible medical expenses. Any deficiency in the budget of a housing authority caused by such reduced rental shall be paid by the commonwealth and paid to the housing authority in an amount equal to the difference between the tenant's rent and the prorated cost of operating that unit. The commonwealth, acting through the department, may make payments in advance on account of such deficiency at such times and in such amounts as it deems proper. The prorated cost of operations shall be computed by the department with provision for a full operating reserve.