You're asking for a numerical value of the odds that she might be pregnant from your having unprotected sex with her.
Yes, there is a chance.
However, given that you were sitting in a hot tub, well after you were coming, the sperm is likely to have been washed away.
But, if there were sperms inside your ureathra, and you penetrated her at all (enven just an inch) you could have deposited them in her vagina. This is what is called unprotected sex, and it's basically gambling.
You are not very likely to have impregnated her... but there is still a possibility. You may be making a bigger deal out of this than is necessary, but the answer is to get an EPT home test and have her take it. That should set your mind to rest.
But from now on, you use protection, every time.