What is the accounting objective which best relates to the % of completion method ?

Accurate presentation relates to the percentage of completion method.

Try Relevance in terms of does the method closely approximate the activity or its driver.

CONSERVATISM. Percentage of completion method is used when.

         *   collection are assured

         *   The accounting system can; estimate profitability and measure progress towards completion.

The principle of  conservatism states that you should always error on the most conservative side of any transaction i.e minimizing profits by recording uncertain losses or expenses and not recording uncertain or estimated gains.