The Rule of 6

If opener repeats his suit, it should contain six cards.   Don't repeat a five card suit - either the hand will be balanced - when you open/rebid Notrumps or it will have a second suit of four cards that can be bid.

The Rule of 7

When holding just one stopper in the suit led (in Notrumps), add up the number of cards in your hand to dummy's. Subtracting the total from seven will tell you how many times to duck.