Question for 9 hc.
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It depends entirely on the courses you're playing. Your hcp index is calculated by taking 96% of average of your lowest 10 of your last 20 differentials. Differentials are based on the course ratings and slopes of the courses (and tees) that you're playing. Golfers playing relatively easy courses will have to shoot much lower scores than those playing very difficult courses.
Do you have an actual, established hcp? If so, then the short answer is take a look at your current differentials......based on the courses you're playing your lowest 10 scores will have to give you a differential right around 10 to get your index down under 10.
Its pretty simple. If you a 13.8 and want to be a 9. Then that is 4.8, basically 5 strokes better than what you are shooting. So you need to find 5 strokes in a round and you will be at a 9. Each course score will be different like mentioned above depending on the rating/slope of each course. An easier way to figure it out is say your home course you normally shoot 83-87 and its rated at 70, then you need to shoot 78-82 to be a 9.
I'm not sure if you are looking for a math answer -- how to calculate differentials and then your index -- or a sense of the scores answer.
I broke under ten index when I started having a slightly better chance of shooting in the 70's than I did shooting in the 90's. I used to save a ball from every round in the 70's. I'd write the score and the date on the ball and save them. I knew I was heading under 10 to stay when I stopped commemorating my rounds in the 70's. Still thrilled to shoot a 79 but not save-the-ball worthy. I got to my lowest index of 6.1 when I stopped shooting in the 90's and I felt like a bad round was 85 -- anywhere. That lasted half a season. Hope to get back there this season.
BTW, a slope of 136 is a heck of course. Consider moving up a set of tees and learning what it feels like to break 80 more often. You can move back and forth but learning to have birdie putts creates habits that can carry forward.
Rusty the last part of your post is great. I overheard a few people the other day saying they practice from the front tees but still use their drivers and all just to work on scoring more. They said it translates greatly when your playing from the backs. BTW the 136 is only playing at 6400.