How does this feel for you:

"I really like you and we get along so well that I am wondering if you might want to go on an actual date. It's okay if you don't! But if you do, I have something in mind."

If it feels okay, then edit as needed so it is in your own words.

What is the probability that she will say yes; what is the probability that she will say no?

Think about this scenario: you never ask her out. You stay friends and she gets someone else as a boyfriend and eventually marries him. How do you feel?