Preempts are meant to disrupt opponents communications with enough tricks in hand to prevent disaster. A good example occurred yesterday. Here is your hand vulnerable vs not vulnerable:
When you preempt it’s necessary to count your tricks. When vulnerable you want to be within 3 tricks of making your bid, within 4 tricks when not vulnerable. It looks like you have 4 tricks in spades and one and one-half in diamonds for five and one-half tricks. That’s too few vulnerable to bid three spades. Open the hand two spades, and you are within the requisite margin of safety. The only downside is that your partner is likely to lead an unhelpful spade if LHO buys the contract.
Bottom line when preempting with a weak hand: count tricks rather than points, and stay within the margin of safety (three tricks vul, four tricks not vul).