Potentially a strange question for this forum, but I find most people on here to be pretty damn intelligent, so what the heck. 
I have a home refinance question that I would love to get advice on. My wife and I currently have a 30 year fixed loan @ 5.25% from a 2007 home purchase when that rate was considered very good 
At that time, my wife had a credit score in the high 680/low 690 range and I was around 760. Since that time, my wife went through a bit of a credit crisis with some of her personal credit cards and… to be frank - hid it from me - so I was unable to assist or control the issue. Needless to say, this all came bubbling to the surface when I started down the refinance path with these historic low rates well under 4%. I have started the repair process for her credit and have been able to get her score up from 550 to just over 600 in the past 3-4 months and my rate is now at 791.
Given that our home loan is over $450,000, that 1.25-1.75% will have an large impact and rates are starting to creep back up. I am scared I will miss the window, but believe (know) her current score will severely impact (kill) our chances at a sub 4% rate.
So my questions are;
Given that there is no way her credit is getting to 700+ anytime soon, what should I get my wife’s credit score to before I even try to secure a refinance?
What rates can I expect with a 791 and 600 score? Are we lucky to just have 5.25% now?
I have heard 620 is the first “magic” number, but will a joint refinance with a 791 and a 620 net a decent rate?
What score would she need to hit to get a prime rate when coupled with my 791?
Any feedback is greatly appreciated.