Well.....
I stupidly shot in the 40s. Do NOT do that!! However, I will shoot in the 50s. However -- and this is a very big "however" -- I have a ton of data, I know my cat and how she responds to high carb food, I have a lot of experience in managing her numbers when they drop, etc. There's a huge difference in shooting low numbers when you have data behind you vs. shooting when you don't know that you don't yet have the data.
We suggest that you do not shoot below 150 until you have some data -- you need to know when Lantus onset and nadir fall. Once you have those data, you can begin to shoot progressively lower numbers. I think I would get general agreement that the first time you test and see that your cat is in the 40s, you're heart rate kicks up, your hands shake, and you start shoveling large quantities of food into your cat (not always the best idea -- the large quantities part, that is) because you're convinced that your cat will die from a hypoglycemic crisis. We've all stood in those shoes and learned how to steer the numbers to keep our cat safe.