You are not required to specify that the goods are capital goods. In fact I will take this opportunity to explain the basics, which will explain the things.
Invoice is for the purpose of doing commercial transaction. You should write on the invoice all those things which you need to write for the purposes of commerce. It is just that on such goods some amount of duty is payable, that has to be calculated. Rule 3(5) of the Cenvat Credit Rules says that when input or capital goods are cleared as such, the amount of credit taken is to be reversed. Thus, the 16 rupess credit taken is to be reversed. The second proviso of the Rule says that if the capital goods are removed after being used, the amount of duty paid is equal to cenvat credit reduced by 2.5% each quarter or part of quarter.
It will shown in the ER-1 as credit utilised on removal as such (para 8, row 12 of ER-1).
There is no assesable value in this case. At the place of rate of duty where you writes ( @ 8%), write [under Rule 3(5) of Cenvat Credit Rules], which will clarify the things.