NOTOC forms are to be a mirror copy of the DGD. The main purpose of the NOTOC is to convey to the Captain what dangerous goods are loaded on his aircraft and their loading positions. Therefore, conveying the information using a calculated average is not totally wrong however it is more advisable to convey the actual picture as stated on the DGD instead of stating 5 plastic jerricans X 5.4L. As stated in the latter part of DGR 220.127.116.11.3 (e), when there are multiple packages of identical PSN and UN number, convey the total quantity, and the contents of the largest package and of the smallest package.
Accordingly, the following entry on the NOTOC will convey all what the Captain needs to know.
5 Plastic jerricans Total Quantity 27L
Largest 11L, smallest 2L