Terri,
With Lantus it isn't so much where they are at preshot as how far down the insulin is taking them at their lowest point that we base the dose on. So cats just ride high all day long and we need to increase the dose, some will fall very fast during the day and bounce right back up at preshot time. That is where testing becomes important so we know how fast and how low they are going. My little girl Autumn (although I can no longer really call her little) was under 6lbs when I adopted her as a diabetic, her BG was 528, and I started her on 1u eventually raised it to 1.25 but that proved to be too much for her once I transitioned her over to an low carb/high protein diet...today she is 14lbs (her ideal weight) and is on a scant .25u and may eventually completely give up insulin altogether. On the flipside of that is my Maxwell..he was 10.5lbs when I adopted him and never went higher than 1u and again once the diet changed he went into remission and has been insulin free for 2+ years now. He is also a large cat by frame and is currently 17lbs also his ideal weight.
As much as I wish this was an easy formula of if the cat weighs x, and eats y, then the dose is z that just isn't the way it works, there are many many factors to figuring out the correct dose and only a very few do we have any control over whatsoever. While yes she will probably need more insulin while still eating dry food without knowing how low the dose is taking her and now with a diet change we would rather be conservative than risk her health by giving her too much insulin and sending her into hypo...High numbers kill over time, too low of numbers kill very quickly.
Autumn went 10+ months without any treatment whatsoever when she was first dxed until I adopted her, but she is still here and now is trying very hard to give up her shots. If you would like to post over on Relaxed Lantus I will try to help you figure out what is best for your little girl.
Mel, Maxwell, Autumn & The Fur Gang