According to WIS, the adjustment is +/-4% to "some" of the players stats, whatever that means. The question you have to answer is, "At what point are the positive effects of formation practice worth more than the positive effects of the individual practice that I'm sacrificing?". Personally, I haven't seen any perceptible advantage related to formation practice, so I spend little or no time on formation practice, and put most or all of my practice time into individual practice. No way to know for sure, though, because even WIS doesn't fully understand how the stats are applied to get a certain result. Just a guess, though, would be that since as you go up in division your rate of stat improvement decreases, formation practice becomes more important as your division goes up. Exactly how important is anyone's guess.