As a consultant I am often put on the spot for specific answers and decisions for very ambiguous and undefined problems. Sometimes they want the finish date for a project, or a how much it will cost to build some gigantic piece of software. Whenever you faced with taking a stand on something that is very unknown you should address it head on: "I do not know."
Many people have a problem being able to admit their ignorance. One of my mantras is often misinterpreted to mean I always have THE answer (Often Wrong, Never in Doubt). What must be realized is that I freely admit I don't have THE answer. That is not the same as not having AN answer. My answer might often be that I don't know, but you'll get it clearly and quickly when that is the case.
The goal with always having an answer is avoid paralysis and continued chaos. The goal with admitting ignorance when appropriate is to illustrate the care and attention to detail that you should have for your decision-making. It helps to be Articulate, but only if you are first Deliberate. So being deliberate about how you provide answers is critical to keeping credibility and integrity.
In addition to showing that you care for quality of your responses, you can show that you care about the input of others by asking questions to clarify what is being asked or what will make a decision acceptable. Take the time to understand what a good answer would be, how precise or concrete the details must be, what is at stake in the decision. Being able to clarify the factors that go into your answers and responses is important for being able to defend or justify your position.
This technique of acknowledging ignorance and seeking to understand doesn't just help the quality of decisions, it can actually help ensure you are making forward progress. Breaking big issues into smaller ones, dissecting tasks into dependant steps, is a great way to not get sucked into Analysis Paralysis.
Even in your personal life, if you have a challenge that seems overwhelming, start distilling it into small steps and milestones. Then you can move forward by focusing on just one step at a time.
At work on software projects we use 2 hour increments and daily milestones. Anything more is just too big to digest. By using small units of work, we stay nimble and can celebrate victories more often. Instead of assigning work that takes days, I assign work that takes hours. It takes more effort on my part, but my engineers aren't able to procrastinate. The productivity can stay very high and because I have to understand the work at a much more granular level, the outcomes are easier to predict.