Linda, some will claim I'm committing bowling heresy, but I suggest you not use the 3-6-9 system. That system assumes too many variables which may/may not exist in different bowlers. Instead I offer up a conceptual approach that can be customized to your delivery. Also, except for a few extreme splits, will allow you to pick up almost all spares with just 6 positions. It will sound like the 3-6-9 but it's not.
Here's the concept; Whether you throw the ball straight or hook the ball, where it hits 60 feet away is controlled by where you end up at the foul line. Visualize a teeter-totter laying on it's side. If you move one end to the right the other end will move to the left and vice versus. The teeter-totter can be straight or curved but if you move the end at the foul line right or left, the other end will always move in the opposite direction.
If you are shooting on a typical house shot, there will be more oil in the middle of the lane than on the outside. If you use 3-6-9 with a straight ball, many of the angles will fail to pick up spares because the ball will skid down the oil. The solution for all pins is to shoot at an angle across the heavy volume in the middle. This may be accomplished with 3-6-9 but experience says it probably won't. 3-6-9 works best for those who throw a hook.
The solution is for you to experiment, keeping in mind where do you need to stand to pick up all the combinations of pins you might leave. For example the same shot that picks up the 3 can also pick up the 3-6, 3-6-10 or 3-6-9-10. The same can be done on the other side with the 2, 2-4, 2-4-7 or 2-4-7-8. In addition these 2 shots will also pick up the 3 pin bucket on both sides. i.e. 2-4-5 or 3-5-6. As you can see, about 80% of all the spares you will leave can be attacked from basically 2 locations. 4 pins and 4-7's along with 6 pins and 6-10's can be attacked from 2 other locations. The only way to know where you need to stand for your ball to pick up all these single pins or combinations is to experiment.
Your strike line may or not provide you the necessary angles based on your delivery. The next time you practice, try this. Bring a notebook
and pen. Forget throwing to get a strike. Instead, stand anywhere you like and throw a ball that will hit the head pin and continue into the 2, 4 and 7. Move your feet until the other end hits in the correct place. Note which board your left toe was on to accomplish this task. When you can repeat this shot 3 times in a row, make a note. Do the same for the head pin, 3, 6 and 10. Do the same for the 4-7 and 6-10.
By the time you master these 4 shots, you'll know if the 3-6-9 system will have accomplished the same thing. You'll also know how to pick up any single pin that makes up any of the combinations mentioned.
Where it gets tricky is when you have sleeper pins. Examples would be the 2-8, 2-4-5-8, 3-9, 3-4-5-9 and so on. These spares require you hit on a specific side from a specific angle to be successful. For example, a ball that hits the 3-6-10 will not carry the 3-6-9-10. You will need to select a different angle to carry the sleeper. Only experimenting will show where you need to start. With a straight ball, you will need to angle you ball from the far right corner of the lane to hit between the 3-6 to carry the 9 and 10. I'm suggesting with simply 6 shots or maybe a couple more, you'll learn to make any single pin or multiple pin combination. It'll take you about 5 games to determine and note where you need to stand for these combinations.
If you watch the PBA or PWBA they all use this system. They do exactly what you do. They take out a ball that will go straight, stand where they know they will have the correct angle and throw a 'straight' ball to pick up all their spares. They may all shoot strikes in almost the same place, but each will have a different place they use to pick up all the combinations.
Four shots will get 80%. Six shots will get an addition 18%. To pick up spares like the 4-7-10 or 6-7-10 will require 2 more shots for a total of 8. But for 98%, 6 shots will get the job done.