So if his stamina is average find the sweet spot, I recommend somewhere between 60-65. You guys are talking about tsi - is that DMI? If it is, don't put too much emphasis on it. it can be very misleading. Strong to proficient GS is not that big of a deal. Even if a player has prof GS he can still suck in a game and subsequently a player with respectable GS can outshine someone who has better GS.
I usually use the first game of the week as a better indicator on how players perform and make adjustments moving forward.
Further, I used to train GS weekly before they nerfed the effect and now we can only do it once in while to be effective. That for me was a better tool to use for in game performance.