The amount of any loan shall not exceed 50 percent of the local agency’s property tax revenues for the 1977–78 fiscal year, including state property tax relief subventions. The maximum amount of a loan to a local agency which was formed during the 1977–78 fiscal year and authorized to levy a property tax, but did not do so, shall be no more than 50 percent of the amount such agency could have levied if it had levied such a tax.