It sounds like you're off to a good start!
First, charge more than just enough. It's amazing how fast the incidentals like targets, staples, unexpected expenses, etc. add up until you're paying for stuff out of your pocket to keep it going. You can always lower the price later once it's rolling and you have kept track of the accounting. That will make everyone happy.
Try raising the price if you come up short and there will be no end to the howling.

Get some range safety help. DO strictly enforce range safety! There is too much going on for one person to call the match and keep an eye on all the shooters. Get a liability waiver form drawn up that each shooter has to sign. Print course of fire and range rules and give every shooter a copy.

If the range has a gun shop, remind the owner of all the extra sales he will make from the competitors buying ammo, holsters, guns, etc.

It can be a fun time for all if done right.