cya level and chlorine

[p.p.h.]

I tested my cya level 2x on different days at different areas of the pool just to be sure and low and behold my CYA has dropped to 40. The owners manual for my SWC says to keep my cya at 80. My free chlorine level is at 10 right now with water temps at 88-90 degrees. Whats a good FC level to be running at or should I just leave it at 10 ppm ? Would it be safe to run a CYA of 40 and a FC level of say 4?? PH is at 7.4 and ALK is 120 if that matters.