Boy, this is tough subject for sure. I've had to go through this twice - once in your circumstances (having to evaluate when) and another time was very sudden.
I worked with my vet and she was more than open to coming to my house when I thought the time was right. Sam was a 15 years old female black lab - had her from 6weeks - and had suffered weight loss, loss of interest in the things (and appetite), and was having trouble seeing and even maintaining stability when walking. Especially on tile floors.
We took one last walk to her favorite field - we laid down - and I balled my eyes out with her - thanking her for all the great times she afforded me AND being my mothers go to soul during her terminal illness. She got a few milk bones too

. When I got back home I called the vet and had her come down. In the mean time Sam went to her place under the window, next to my bed and went to sleep. When the vet arrived, she gave Sam a sedative to keep her calm (she actually never woke up all that much), and then put her down. I held her as she took her last deep breath and it was over.
Very peaceful, and I knew it was the right time. You will too - even if you don't think so now.
I'm so glad things happened at home.
Best of luck - it's a hard choice - but the fact that you even chose to ask advice speaks volumes for the type of pet person you are, and guarantees that you'll do right by her.
PS - Atlanta Man, sorry to hear.