If you are following the SLGS protocol, then the recommended starting dose for a cat on Lantus - if the cat is on a low-carb wet food diet - is .5 unit. every 12 hours.
I am not sure if you have decided on which protocol to follow. If it were my cat, I would not start with anything larger than that. In fact, and this is just a personal opinion, but I might even start with .25 units because I see a lot of yellow on Merlin’s spreadsheet- so out of an abundance of caution I would probably so that with my own cat. It’s not what is written in the SLGS guidelines, but this is what I probably would do with my own cat. I would also feed my cat before taking it to the vet.