It's a fairly individual decision, but here are some things to consider:
1. What alternatives do you have if your plan starts to look like it might fail? The more alternatives or backup plans you have, the lower a success rate you might need to feel comfortable.
2. What is your opinion about the future? Firecalc is based on historical data, and 100% means you would have survived the worst of economic times in the US. Perhaps you think things will be better. Perhaps you think things could be worse. Perhaps you don't think the past applies. Perhaps you think the US is a special case.
3. How much of your retirement income is "secure"? The more you have in pensions and Social Security and the more secure you consider those to be and the more they cover your basic cost of living, the lower a success rate you might be willing to accept.
4. How much of the spending that you put into FIREcalc is required versus optional? This is a variation on the first item, but the more of your FIREcalc spending is optional, the more you can handle a bad situation.
5. How many people depend on you being right? If you have dependents or heirs, you may choose to be more stringent in order to protect those other people.
For me, the answers are:
1. Lots of contingency plans.
2. I think the future will be economically better than the worst of the past, and I think the US is a special case that will endure in the future given certain criteria.
3. I don't have a pension, but my SS when I hit 70 looks like it will cover my current expenses with a little bit left over.
4. I put in my actual spending in Firecalc, and as a relatively frugal person, most of my spending is required, not optional.
5. I have no spouse, but I have three kids. But they're in the process of launching into adulthood and will probably do well without any further support from me (other than paying for college, which is saved for in a different bucket).
So for me the mix of all five of those factors means that I am willing to spend up to a level which Firecalc would consider 95% historically safe. In actual fact, my spending is currently about 40% of that number simply because I don't need to and don't have any good ideas for ways to spend money without being wasteful.