Breaking down dog antibiotics for a cat

I have 400mg/100mg clavaseptin ABs unused from my working dog. Looking at the drug information for clavaseptin a cat of 4kg would require 50mg/12.5mg tablets for an infection. I suspect my cat has an upper respiratory infection of some sorts he’s fine to wait until tomorrow for the vet but I thought surely it won’t hurt to get started on some ABs to hopefully help knock any potential infection back and make him a bit happier. Am I correct that 1/8th of the 400mg is equivalent to 50mg for a cat? My normal patients are cows and horses not cats