The guys already have given you exceptional pointers on where to gather your knowledge, but I have a few things I want to plus you up on.
Ultimate Sniper is God's gift. I have it in place for 'BRM' (not Basic Rifle Marksmanship
) so when I take my morning glory I thumb through a couple pages. I have read it cover to cover a coupe time but with the amount of information available in that book it is easy to forget or miss something. Several friends refer to it as 'The Bible' and at the risk of sounding sacrilegious, I am inclined to agree.
Another thing that guys get hemmed up on is the use of laser range finders. Unless I have a good backstop within a meter of the area I am lasing, I won't use one. When you start reading 'The Bible', you notice that he hits on the use of range cards similar to the ones you would use for 240's. That is the time to use your range finder, when you are drawing up your range card. It all comes back to the basics. Use terrain association from your map (given you know your present location, I hope you do), section and re-section, and your other map reading skills. There is a reason that for any selection that you go to (scout/sniper for instance) pushes map reading and land nav so hard. Because it is the basic go to tool for so many other reasons than to find your way in the woods.
I can tell you another trick me and the dudes used to use in Iraq to solve the unknown variable of how big your target is. As a previous QP stated, 1 mil is 1 meter @ 1K meters. The variable is how big IS your target? The average male is 68", but that is what it is: and average. Use your environment. Is there a Texas barrier near the target? Those are 80" if I recall. That is a fixed variable that is not subject to change. So, if a 80" barrier is 1 mil in your scope, and you know that 1 mil is 1 meter at 1000 meter, convert the inches to meters and do some math. Then your dope book (or a smart one would have a card) will tell you how much to come up. Use the environment.
The role as a DM as opposed to a sniper is much different. The only constant is the shooting portion. Every single time I filled a role as a DM I did not have the time to set up a range card, lase targets, find a good hide, etc. I had to rapidly engage threats. My training in 'stress shoots' played a pivotal role. Train to rapidly asses your targets, identify the range, and engage in quick succession. Train shooting from awkward stances. You will find that you will almost never shoot from the prone or benchrest. Make a piece of plywood that is way too high to shoot from the knee, but way too low to shoot from standing. Practice off of that more than anything.
The study of sniper tactics will enable you though. Because many times, you are working as a counter-sniper. You need to understand where the threats would logically come from. If you were to attack your patrol, where would you shoot from? Study sniper tactics not so much for your own tactical application, but to help you engage those threats that work with that manner.
Spend the $60 bucks.
One other thing I forgot is make a sand sock. You can read about it and the use in the book.