Rule of 17 responding to weak twos
The "Rule of 17" is a bidding guideline used when your partner opens with a weak two bid. If the number of high card points plus the number of cards you hold in your partner's bid suit equals 17 or more, you should bid game.
This guideline will apply the more your partnership has discussed what to expect when partner makes a weak two bid. If you are a very aggressive pair, this might lead to some troublesome game contracts down a few doubled. If you are a very conservative partnership, maybe times as few as 16 or even 15 could result in a very makeable game contract. The takeaway is that the Rule of 17 is just a guideline. A paint-by-numbers rule, and it cannot replace bridge judgment. However, it is a good evaluation for you to 'think' about how to respond to your partner's weak two bid. Remember, standard Goren point counting has a few defects. Undervaluing Aces and overevaluating Queens and especially Jacks. It also ignores the value of connecting honor cards. A good 2nd check to aid your judgment would be to look at the rule of 2/3/4. Opener should preempt with a hand that expects to go down no more than 2/3/4 based on the vulnerability. You can evaluate your Losing Trick Count opposite such an assumption. This calculation also helps you determine when a higher level sacrifice can/should be made.
Let's look at a few examples where the rule works. For discussion, our partnership makes 'moderate' weak two opening bids. That is defined as: * 6 cards promised * won't have 4 cards in other major, won't have a void * Minimum 5HCP, Maximum 11 HCP. If only 5 HCP, it will be KQ or AJT in the bid suit * Avoid very light weak twos when vul vs non-vul
In both of the above cases, 10 tricks seem a likely possibility if partner has a reasonable weak two opening 2♠ bid. If you have some more tools in your toolbox, bidding 2NT to inquire about the opening hand is useful to talk you into or out of game level.
This hand only has 12 HCP and 3 spades, making 15. However, do you want to be in game with it? Of course you do. The void and touching honors are not correctly evaluated with simple HCP. You can expect your partner to have an 8 or 9 LTC, 6 spades. They will very likely have the A♠ (but that depends on your style). Preempting with JTxxxx suits and the bottom half of the 5-11 HCP range is not a strategy for long-term bridge success. You can also predict/expect that partner will not be void in hearts. The 9 card trump fit and moderately good connecting hearts should produce a few additional heart tricks. You have first round control of the diamonds, which will score your 2 of trump as an additional trick.
Here is a hand that scores 17 (16 HCP plus "1" for that singleton trump). However, after 'considering' the hand, my judgment would be not to bid the game contract. This won't play well in 4♠. It might have a chance in 3NT if partner has a good spade suit (two of the top three honors) AND the top half of the weak 2 range. To determine that your partnership might play OGUST conventional inquiry to ask about the quality of the weak two open. However, if you are not playing that, consider a few other things with this hand. First, you'd love the opponents to jump into this auction at the 3-level; don't deter them. You side as at least half the HCP, and opponents will not get a favorable trump break here. Doubling 3♥ or even 4 of a minor might get you some gravy.