We also had Lerwick on TOTW, both the Pacific Stream and the Lamb one, and switched him to Acana Pacifica (made by the same company as Orijin) back when the big Diamond recall hit; we've been feeding him the same amount of Acana as we were of TOTW, and his weight hasn't changed at all. Incidentally, he's 16.5", and a very lean 24 lbs, so it sounds to me like Missy's weight is right on the mark.
I agree, keep feeding Missy the same amount you were feeding TOTW, and just weigh her regularly, to make sure she isn't gaining (or losing) weight on the new kibble. That way you don't have to try to go by the rib method. If your vet is happy with her weight as it is, then you can use the scale to keep her on track.
We weigh Lerwick regularly too, because he's right on the cusp of his heartworm meds (he weighs 24 lbs, his heartworm medication is for dogs up to 25 lbs, and we don't want him going over without us knowing it). All we have is a bathroom scale at home, not really big enough for him to sit on (if he would even sit still long enough to get a reading), so I get on the scale to get my base weight, then pick up the dog and weigh again, and take the difference for the dog's weight.
I agree, keep feeding Missy the same amount you were feeding TOTW, and just weigh her regularly, to make sure she isn't gaining (or losing) weight on the new kibble. That way you don't have to try to go by the rib method. If your vet is happy with her weight as it is, then you can use the scale to keep her on track.
We weigh Lerwick regularly too, because he's right on the cusp of his heartworm meds (he weighs 24 lbs, his heartworm medication is for dogs up to 25 lbs, and we don't want him going over without us knowing it). All we have is a bathroom scale at home, not really big enough for him to sit on (if he would even sit still long enough to get a reading), so I get on the scale to get my base weight, then pick up the dog and weigh again, and take the difference for the dog's weight.
Feel the backside of your hand you can feel all your metacarpals (long bones in your hand) that is what your dogs ribs should feel like with a good weight with the indentations. 


The trade off is that it has lower protein.