(taxpayer may use any reasonable method, including exact method, to determine amount deductible as qualified residence interest when debt exceeds acquisition and/or home equity debt limitations; if exact method used, taxpayer need not make election under Treas. Reg. 1.163-10T(o)(5) in order to allocate interest on part of debt that exceeds qualified residence interest limitations; if simplified method used, taxpayer may allocate excess interest under interest tracing rules without making election).