I understand it but not sure I can properly explain it, but here goes.
Let's say you want to play straight up the ten board on that 41 minus 31 pattern. The laydown point of the ball will have to be the ten board. The average man's width is about 7-8 boards, so that means that the sliding foot will be that far from the laydown point. Assuming RH bowler, that means to lay the ball down on the ten board, the sliding foot will be about 7-8 boards left of that, or on the 17-18 board.
Don't forget that if you are factoring in where to stand on the approach, you have to take into account your drift factor. I don't drift, so mine is zero. I think if you drift two boards right, then you have to add 2 to the 7-8, so your starting point on the approach would be on the 19-20 board in order to end up at the 17-18 board so that the ball laydown will be on the ten board. If you drift left, subtract that number.
If I am incorrect on this, someone here will correct me.