I do entirely - hence the Boost die. However, Difficulty in ranged combat isn't set by a target's speed: it's set by range, which would be reflected in the sniper's distance from his enemy.
The rules suggest that effects that benefit a character translate into Boost die, whilst hindering considerations (wind speed, direction, cover or hindering landmarks/layout) would translate into Setback die.
I don't disagree that your method is certainly valid, of course. But the above is how I would run it.
Oh yeah. I'm not trying to say that my way is best either. Just trying to clarify. In this case, my feeling is that, unless you give the character more boost die than they'd get from aiming normally, their effort to snipe from the shadows is essentially token. It doesn't sway the statistics in their favor (versus regular combat, at least), and I think it should, at least to some degree. (But again, that's just my take)
Edited by Otzlowe