I should add that when I used converted times I found something interesting. Above I noted that I added .3s for every foot I brought the hurdle in. One foot multiplied by ten hurdles is ten feet, or roughy 3m. Since a meter in boys sprints is typically worth a tenth of a second, you’ll notice that even though split times are faster for reduced hurdle spacing, the athlete’s speed is no faster. Thus, when you are bringing the hurdles closer together, you might get them the sensation of faster split times, but they are not running any faster. The only way to get the developmental kids to run faster is to drop the hurdle heights. I know Carl has brought up this idea before.
What I typically do is to move use discounted height and length between hurdles at the beginning of the season, and then progressively move them out as I get faster and more able to handle the increased stress it may add. I’ve also recently adopted keeping the first hurdle at race height at all times because of the different trajectory needed (and also for me specifically because my hips are a little low and I need to raise my center of gravity a bit on my approach to take the first hurdle correctly). Typically you can race having only hurdled with discounted or lower hurdles simply because the adrenaline makes up for it.
Out of curiosity, how much do you reduce spacing by? I still find it hard to believe that it affects an athlete of your level, but I won’t knock what works for you.
Also, I am of the strong opinion that some athletes can adjust to races with less practice at real height better than others. The 5’10” girl mentioned above would jump and jump, no matter how much she loved to hurdle. But after running a couple thousand hurdles at 30″ she just ran the same over a 33″ hurdle. Conversely, the 6′ boy mentioned above cleared every hurdle by a couple inches. It bordered on insane how close he would get to a hurdle. But every time I bumped a hurdle up, there would be some crashing before he found his rhythm again.