The following apply to determinations of the ad valorem tax bond allowable rate:(1) "bond allowable rate" means the portion of the maximum authorized tax rate available for debt service;(2) except as provided below, and except for good cause shown, all political subdivisions have a bond allowable rate equal to 2/3 of the maximum tax rate authorized by law;(3) home rule cities and general law cities authorized by §5 of Article XI of the Texas Constitution to levy a tax of up to $2.50 per $100 valuation have a bond allowable rate of $1.50 per $100 valuation, unless the tax rate is further limited by the city's charter;(4) counties have a bond allowable rate of $.40 per $100 valuation, plus an additional bond allowable rate of 1/2 of the tax rate voted for "further maintenance of public roads" authorized by §9 of Article VIII of the Texas Constitution up to a maximum additional tax of $.075 per $100 for public securities payable from that tax;(5) annual tax funds available for debt service shall be calculated assuming a collection rate of 90% or a higher rate based on the average collection rate for the most recent three years, as certified by the issuer. In calculating the average collection rate, the annual collection rate used for any year may not exceed 100%; and(6) the bond allowable rate may be further restricted by state law.