95% based on your assumption she didn't have sex with any other guy during those weeks. Since you know she has been with other men, please for your sake, don't assume anything. There are at least two other men you know of, not to mention ones you possibly don't know of. In reality, that 95% is probably much lower. I know part of you wants to be very excited about this, but until DNA testing has been done and you are confirmed the father, you should be taking an analytical approach to this and not an emotional one (I know, harder to say than do), but for your own peace of mind, it is best not to emotionally attach yet.
You have gotten alot of great advice, the most important being, until the baby is born and you are confrmed the father through DNA testing, you don't agree to anything legally. Once testing is done and you are confirmed the father, then you start to deal with the legal process of parenting time, etc... You can't do anything legally until the baby is born and the first step following that is getting testing done. Do not sign any agreement regarding paternity, visitation, support, ...until then. you need to be highly skeptical of any attorney who is trying to get you to do this while she is pregnant or of her for trying to get you to do that. That sends huge red flags up to me and it should to you as well.