How would I make this if statement? (if somehting is within x amount?)

Using Arduino Programming Questions
48

I've lost track of what you're trying to do, but this is a problem:

 if ((delta > 00.05) && (delta < -00.05)) {
    digitalWrite(13, LOW);

delta will never be greater than 0.05 and less than -0.05. You could use or (||) instead, though it looks like you should just drop the if adn make this an else clause for the previous if.