Family, religion, math fuel Spurs’ jersey selections.

David West leaned back in his chair and sighed before revealing the formula behind why he wears No. 30 on his jersey. “Fifty minus 21,” said West. While the answer to that subtraction problem is 29, West borrowed an invisible one and produced his magic number — 30. Fifty and 21 have special meaning to West. “Tim (Duncan, 21) and …