How can the percentage of completion method of accounting be applied to construction jobs that have not begun?

This is typically not provided as the project has not commenced.
However, based upon soft costs a percentage completion can be tallied based upon overall costs in proportion to soft costs only.

In order to verify percentage completion from accounting to verify actual hard costs complete you can utilize the y=sin2 (90-x) method.
Basically utilizing a grid spreadsheet based upon number of months in a construction schedule with each block evenly distributed for percentage complete. E.g. a 10 month construction schedule, after 5 months the project should be reflective of 50% completion, thus the same for accounting G702 G703 summaries.