Excise duty calculation mostly falls under 3 categories based on the Tariff:-
1.Specific rate of duty - sugar ,cement etc are charged at specific rates irrespective of sale vaule.
2.Transaction value - This is the basic sale value plus debit notes etc(sec 4)
3.MRP basis - certain products are charged on retail sale price basis after allowing certain abatement in MRP, irrespective of base sale price.(sec 4A)
The method depends on the chapter in which it falls. There are other methods in Central Excise Valuation Rules when there is no sale involved or where the above methods are not applicable.