trent,
i believe usage of A/T during a manual landing varies from company to company. having said that, i believe boeings' procedures recommend the usage of A/T.
usually the FMC does the calculation for TOD
However, if you have to do it manually, it's Altitude (thousands of feet) X 3 plus 19nm. it is available in the QRH (if you can still remember how to read it
)
SR