...I don't like having the hard 50% number for how much to practice putting. The goal to hit everything inside 20' is spot on. Practice as much as you need to to meet that goal. One of the observations I have made at tournaments is that putting can be the difference between a good intermediate player and an average pro.
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Here's something I like to point out to emphasize the importance of putting. If you double your high percentage putting range, say from 10' to 20', you quadruple the area you have to hit. How long do you think it will take to go from 10' to 20'? How long will it take to get four times more accurate with your driving/approaching from all ranges? Tripling it makes the area you have to hit nine times bigger. Nine times.
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I've given this number of 50% of your practice time allocated to
just putting practice a little more thought, so I wanted to throw this out and see what everyone thinks.
I think that the hard 50% number is there to stress to
new players the importance of finishing the hole. By spending so much time on this one aspect of play, the new player actually feels more at home as they are holing out than anywhere else. The ability to make putts from 5' to 10' further away than your competition increases your drive's landing area exponentially as garublador explained above. I also think that putting practice is easy on the body. No one has ever that I know of thrown their shoulder out or wrecked a knee/elbow from putting 20' out. Stressing the importance of this allows the newer players to put a lot of themselves into improving at the game without overdoing it physically(remember, they are new, and very passionate about the game). Psychologically the confidence that is gained by watching 15'er after 15'er bang in is
HUGE when you are just starting out.
I do want to add that as you progress in building your discgolf tool belt, you will eventually want to spend more time on the other aspects of the game. For myself, 2011 is the year I'm adding "thumbers" to my tool belt of shots to choose from. I practice them in moderation due to the strain it puts on my shoulder, but I have already thrown 4 thumbers in tournament situations in 2011. Without the practice time spent in my yard throwing these, I never would've had the confidence to trust the big tall route over the trees when it mattered the most.
While the 50% hard line number for Rec players may seem like a bit much, I would like to ask you in closing: Who would all of you rather have as a doubles partner? The guy that can throw 280' and misses almost every putt outside of 15', or the guy that only throws 240' and makes almost every putt inside of 25'. When I first started playing this game almost 4 years ago, I was the 240' guy, and pros loved having drawn me as their Pro/Am partner because even though I wasn't much off the tee box, they could pretty much just leave their putter in the bag and just clean up after my misses.
Longwinded as always, but I hope this helps clarify why I believe in and recommend the 50% number for new players. Thanks for reading.