Which approach is used when existing period values are sufficient for future projections?

The breakback proportional approach is utilized when existing period values are deemed sufficient for making future projections. This technique allows for forecasting future values based on the proportionality of past data. By maintaining the same ratios or relationships observed in the historical data, this method generates projections that are effectively grounded in actual performance. This approach is particularly beneficial when trends and patterns from historical data can reasonably be expected to continue into the future, thus aiding accurate forecasting and planning.

In contrast, the other options do not align with the concept of utilizing existing values for projections. The breakback evenly method would imply distributing total amounts evenly across periods, which may not accurately reflect trends. Sheet access control pertains to permissions related to data visibility and editing, while user-assigned sheets focus on the organization of data across different users, neither of which are concerned with the method of forecasting based on historical period values.