No engineers needed. Every other sportbike factory specs are much looser, us track folks run it even looser, and there are no issues, only benefits. Full rear suspension movement, etc.
You should be at least 1 inch or 25mm. I run 2 on all my track bikes I've ever had.
For example, the R6 runs 30-45mm (1.18" -1.77") of slack: https://www.manualslib.com/manual/612270/Yamaha-Yzf-R6.html?page=79
It's like Yamaha forgot the 1" in front when they made the FJ09 manual.