I'm so sorry to hear that Dusty's time has almost come. I know that you're doing everything possible for her and you don't want to see her suffer. You'll know when her time has come. She'll let you know. My RB Pepper let me know by the look in his eyes and he was trying to tell me that he had enough.
I've only had to do this once and it's a very hard decision but I could never have an animal continue to live in pain or suffering just because I wanted to keep it alive. I hope that this makes sense. Just give her lots of loving and comfort with the time that you have left. I'm sure that she knows that you love her and are doing everything possible to make her days comfortable ones. May her passing be a smooth journey to the Rainbow Bridge. Please take care. (((HUGS)))