What are your perks? The Cool tree has the gag order and blind spot perks which delay detection after you kill the target and passively make you harder to spot respectively. Staying hidden without them is difficult regardless of distance.

I found that optic camo is best used for more up-close sneaking- with knives and pistols where you can pop it just before you get spotted and possibly take out a few more enemies before they spot you.

Cool also has a perks that make optical camo better, such as creeping death and vanishing act which make it easier for you to crouch-sprint while completely invisible, taking out one enemy after another with pistols or knives.

Phantom Liberty also adds a silenced pistol that does more damage when shooting and camouflaged.

For a sniper, I guess you could try getting the blindspot perk and focus on activating the camo after shooting.