finding your coords becomes pretty easy once you get a boat and go around a lot more. Before that it doesn't really matter anyway.
The damage from a sling can be found on the
rob-wiki:
Max damage = 60 * sqrt(ql / 10)
Min damage = 0.5 * (PER / AGL) * MAXDAMAGE
so it scales with the QL of the sling as pretty much anything else (square root of the QL divided by ten).
Ranged combat goes like this: you charge your shot (the colour of the bar giving an indication of the hit chance).
When you hit, the RNG generates a number between the min and max damages to inflict on your target.
also, gratz! Which graphical mod is that?