I wont give u a specific number. What i will give u is how to figure out YOUR sensitivity based on a guide i found helpful.
Your scope flick should rotate u 45 degrees
Your unscope flick should rotate u 90 degrees.
I personally use linear ramp though ive heard expo is better for tracking. But im more of a flicker on most cases.
I have a mouse that i can change dpi on the fly and i generally use higher for match ups that i need to flick alot (genjis, tracers, lucios alot more close ranges) and lower it for tracking (lots of far ranges and slow heros, ohara, zen, mercies junkrats, mei).. depends on the map and enemy team com ....
I tried dot for awhile.. it got hard to see.. making it bigger blocked me alot. I now stuck to no gap white cross ... Not too long and thinnest. But some ppl prefer to have a spacing about headshot size. Can give that a try.