As stated in DGR 220.127.116.11 Ullage, sufficient ullage must be considered in filling a drum or a jerrican with liquids. Pressure at sea level and the pressure experienced in the cargo hold of an aircraft at cruising altitude would show a pressure differential of about 25kPa. It would theoretically mean that a 25% ullage would be desirable.
Needless to state, for air transportation, it is a prerequisite that the packaging material must be of such strength that it would endure a pressure differential of 95kPa. As stated in DGR 18.104.22.168 (c), the contents of the inner packagings must not exceed the inner packaging limits of the Packing Instructions shown in columns G, I and K of the List of Dangerous Goods. The quantity inside the packagings must not exceed the design limit of the packaging itself.