I've seen a drill like this for the last couple of years and always felt uneasy about it. It just didn't pass the smell test. The other day, I went out to take some pictures and measurements and did the math on it. Here goes.
Here's how I usually see the drill set up. The quarter on the left is about 1 foot (30cm) away from the dime on the right. The drill then states that if you can roll the ball over the dime (diameter of .705 inches or 1.8 cm), you're start line control is satisfactory. Certainly the distance the dime is placed can vary. I've seen 10 inches to 18 inches away but 12 inches is the most common distance I've seen.
This image is a close up of an estimation I made. Because the golf ball is heavier than the dime, it sits lower in the grass. This causes more places on the ball that can touch the dime aside from just the bottom. The green lines represent the center of the ball on either side of the dime where no part of the ball no longer touches the dime. This spacing between the green lines is roughly 1.5 inches (~3.75cm). This means that if the player can roll the center of the ball between the two green lines, the dime will get hit thus passing the drill.
Well, here's the math on what was truly accomplished. To be fair, let's say I was too generous with my markings and testing procedure so I'll use a smaller spread of 1.25 inches (3.175cm).
From 12 inches (30cm) away to a spread of 1.25 inches we can use these figures and some right triangle trigonometry to find the angle of the start line window. Doing the math and looking up on the tangent tables gives us an angle of just under 6º.
6º!!!!!!! That is a huge angle for start line. That's 3º either side of the center of the dime. For me, that does not pass for satisfactory start line control at any level of golfer. From 10 feet, a 3º miss would miss the edge of the hole by 4 inches for a straight putt. That's a big miss. My goal for my players is to get within 1º either side of their start line. I think a fair goal for most club players would be +/-1.5º.
Let's figure out where the dime should go to produce this 1º either side tolerance. If we assume the same 1.25 inch window around the dime, the dime would need to be almost a yard away (35.8 inches). So, if you rolled a putt that hit a dime 3 feet (90cm) away that would be very good start line control of 1º either side of the center of the dime.
If we expand our tolerance to 1.5º either side of the dime, the dime would be placed 2 feet away (23.8 inches). This would be a better drill for players during a group clinic of members or something like that.
To be fair, let's also put down some math of where I think this drill idea came from. If we take the actual diameter of a dime which is 0.705 inches (1.8cm) and run the numbers again, at 13 inches we see a 1.5º spread for either side of the dime. This may be where the drill idea came from. It's just that in the real world, the spread where the ball will hit the dime is greater than just the actual width of the dime.
In summary, for most green speeds and for club players, if you use this dime drill make sure you place the dime at least 2 feet away and up to 3 feet away depending on the skill level you wish to work toward.
Thanks for reading and please share if you feel others would benefit from reading.