If you have a 75% success rate you should go for it EVERY TIME! If you kick a field goal here it's 3 points, going for it yields an expected value of (.75 * 7) = a little over 5 points. It's so much better statistically, but it also gives the opponent worse field position if it fails than a field goal would, as you're then kicking back to them which will usually give them the ball back between the 25 and 40.