Question cannot be answered accurately.

It may not be able to be answered accurately due to varying field shapes and conditions, but IA State took effective field capacities with various tool widths across multiple farms and determined a field efficiency based on the averages.  For a 35' header data shows to use 75% of Theoretical Field Capacity for the average field.  An extremely large field is likely to be higher whils small cut-up fields are likely to be lower.  Given no additional data I would say 75% is the average.