You can train a specific skill as much as you want. What's imperative efficiency wise is the average drill quality. The training efficiency decreases with every incremental +20 % starting from average quality of 40 %. So basically, it takes less rest packs to train a specific drill from 40-60 than 60-80. The efficiency decreases drastically when the average drill goes over 120. Most managers seem to stop training a specific drill either at 120 or 140. At that point though, you could still have a specific skill well over 180.
Take "Pass, go and shoot!" for example. It targets speed, passing and shooting. At an average of 120 for the drill, you could have the skills distributed 270, 45, 45. It would be as efficient in terms of rest packs as 120, 120, 120 for each skill.
The training calculator will provide you with all this info. You need to combine different drills for the most efficient training. After you've finished a drill by reaching 120 or 140, switch to another drill.
For example, if you want to increase your ST's shooting as much as possible train "Pass, go and shoot!" first until the average drill quality is 120-140 depending on what you prefer (you can go all the way to 180). The switch drill with a lesser average quality that targets shooting as well, maybe "Shooting technique". Repeat until 120-140 and then switch yet again to another that still targets the desired skill.
Here's a pic of my AML which I've done this to. By training all relevant drills up to 120, he now has an average of 150-170 for most drills that are relevant. As you can see, he's between 180-200 for several skills by then. As I said, it's all described and illustrated in the calculator.
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