DGR 188.8.131.52.3 states that the UN specification marks do not always provide full details of the test levels, etc. and these nay need to be taken further into account, e.g. by reference to a test certificate, test reports or register of successfully tested packagings.
A PG I packaging tested for products with a relative density of 1.2 could be used as a PG II packaging for products with a relative density of 1.8, or for a PG III packaging of relative density 2.7, provided of course that all the performance criteria can still be met with the higher relative density. The 1.5 and 2.25 factor, therefore is not a ratio as face value, but the details have to be proven and/or supported with a test certificate or test report.
You must always check the test certificate or the test report where details of the conditions of use are described.