Hello my friend,
It happened to read some old posts about box deff. and I came to conclusion that if it was a dead tactic it wouldnt be in the game. box tactics calculate the best defender through common sense (OD ID SB) and the best offensive player (JS DR IS) before the game starts and adjust the defence. In your occasion what i assume is that you had to guard a 200k salary player with players 1/4 and 1/3 of his salary. So as in any other deffensive tactic box def works the same way. you need to have the right circumstanses in order to work