FTP Improvements

I know there are variables and other things to consider but something I was thinking about last night. What do you guys and gals think the lowest %of FTP is you can workout at and still make positive improvements to your FTP? I get we are all different types of racers and cyclist and have been through various training plans. So not trying to over think it it’s just a general thought and a topic for discussion. You could definitely argue all percentages improve your ability for sure.

I feel like based of what workouts and plans I have done in the past I feel like with no real scientific proof the 75-80% FTP I saw some good gains for my style of racing(marathon mtb). What do you all think??