I usually play as assault, but a few tips for recon:
Your bullets are affected by gravity, they will drop with distance and take time to reach targets at longer ranges. Play around on the test range area to get a feel for how each rifle shoots at varying distances and how much you need to adjust your aim to compensate.
During play against live targets try to gauge how much you need to aim ahead of a moving target so your bullet will land and not fire too far behind or ahead. Each round fired is a tracer, so it will be easy to see the trajectory of each bullet fired and how to adjust for the next shot.
Whenever possible try to be prone when sniping, it will increase your accuracy, stability and you present as a smaller target. Your scope creates a glint that other players can see that reveals your position, so be aware that you are not invisible.
As far as zeroing goes, certain rifles have a 4th perk specialization that allows for zeroing your rifle when using scopes (this does not work with iron sights). Semi auto rifles do not have this specialization.
To adjust zero use the key or button you use for changing firing modes on your weapons. The current zero distance will be displayed by your ammo counter. You can cycle through it as needed to adjust your zero.
To know how far away your target it, use your spotting scope tool, it will display the exact range your target is away from you. So switch to the zero distance that closest matches the actual target distance. You will still have to adjust for gravity and target movement, but much less so.