"It's not snooker unless you are doing all sevens!"
Okay, don't take my advice as serious
I just adore snooker and trial with some pretty competitive friends. Nothing like trying to out-do each other!
Mandy explained it well, although the points needed actually doesn't change by level (at least in USDAA) - you always need 37 points. The closing adds up to 27, so you need to get a minimum of 10 points in your opening. 3 reds gives you 3 points, so you only need your 'numbers' to add up to 7. Of course, more points = higher placement.
If you have a dog who is good about staying with you around a course, snooker is your game. If your dog tends to seek out obstacles on her own - you need to be a bit more careful planning your course! Meg is a killer snooker dog - I can call her off just about anything, so weaving between a bunch of obstacles to get to the high point ones is easy for her. With Gusto, I have to force myself to focus on making a nice flowing course, because I don't want him stressing about me pulling him off things. You'll have to figure out what Lucy needs from you and the course.
With Meg, I'd totally do 3 sevens on that course. I'd lead out to the red by the chute, and release her with myself between that red and the tunnel on my right. Front cross and run her behind the tunnel (her on my right, so I'm not opening up the other red in the middle). Seven, back to the red in the middle, seven, red near the start line, seven, and into the #2 tunnel to start the closing.
With Gusto, I'd do the red right at the start line, chute, red by the chute, 5 jump, last red, 7 jump, and then into the #2 tunnel.
Remember that if you use the #2 tunnel as your last obstacle in the opening, you need to go back and take it again to start the closing (ask me how I know...).