~Rule of 8

The Rule of 8 is a means of deciding whether to bid over an opponent’s 1NT opener. The key to this system is distribution; overcaller should hold a 6-card or longer suit or two 5-card suits (rarely make a bid with 5-4 shape). Long suits in your hand create short suits that reduce the effectiveness of the enemy’s high cards.

You determine whether or not to bid by adding the number of cards in your two longest suits and subtracting from that total the number of losers in your hand. If the resultant is two or more, bid if you also hold 6+ HCP.

Determining Loser Count
Count only missing As, Ks, and Qs. There are 12 possible losers, three per suit. However, there can be no more losers in a suit than the number of cards held in that suit. For instance, in this hand
S A3
H —
D QJT876
C KT987

There is one spade loser (the 3), no heart losers (you will trump hearts), two diamond losers (the missing A & K), and
two club losers (the missing A & Q) for a total of five losers.

There are 11 cards in clubs and diamonds, less 5 losers, for a resultant of 6. Since you have 6 or more HCP, you should make the bid that shows the appropriate two suited overcall. Some conventions that allow you to show a one-suited or two-suited hand are
~Cappelletti/Hamilton
~DONT
~Brozel
~Astro
~Meckwell