This probably isn’t going to be a particularly satisfying
answer for you; but you need to learn about your character’s profession. This
is a mandatory step when you’re creating, nearly, any character. The old advice
is to, “write what you know.” The restrictive way to interpret it would be
thinking you can’t write someone fundamentally different from yourself, which
isn’t true, you simply need to do some research, and learn about who your
character would be.
In this case, that means looking into the mindset of
snipers. There’s a fair amount of non-fiction material on the subject out there.
Offhand, Chris Kyle’s autobiography, and a couple books from Nicholas Irving come to mind. These are blind recommendations, I haven’t read any, but, they
should help you with understanding the mindset of a modern sniper. Obviously,
if your character lacks a military background, then these books might not be
exactly what you’re looking for, but it should point you in the right
direction. There’s also a much wider range of literature on the subject, if modern
day US Special Forces really isn’t on point.
Depending on what you dig up, the answer may be as simple as
simply running the math, adjusting, and then putting a round out there, without
any real reflection on what that bullet is doing.
The best source of information would be people who have
actually been snipers (or done whatever job you’re researching). You may need
to parse out and analyze who they are as a person from what they’ve written,
but they would be the ones who knew what they were thinking.