Image overlap if output is only a orthophoto

A customer asked me for a orthophoto over a big industrial area. To spend less time in the air and make the processing faster I don’t want to have a image overlap that are higher than required. When the same customer buys volumes measurements I usually have an overlap of 85% and 70%. But I guess the required overlap for a orthophoto would be less? 

 

 

Someone must know the recommended overlap if the required output is only a orthophoto. It must be less than whats need for a volume measurement. 
I don’t want to take unnecessary amount of pictures since it will prolong the flight time and also make the processing very time consuming. 

 

I would go with 85/75. You can always use less pictures when you start to process the project. I always fly with high overlap and reduce the number of pictures afterwards specially when the flight area is far away

Okay. How do you reduce the number of pictures used in the processing?