It takes 5-7 days for the "depot" to fill so you're not going to see the true result of the .5 dose until the depot is full.
Lantus is a depot insulin....that's kind of like timed-release. Very basically, it's like when you shoot the 1st time, a tiny bit goes to work during that cycle, but the rest goes "in the depot" and then slowly releases back into the bloodstream....as you continue to give shots, the amount in the depot starts to fill and can "release" a larger amount into the bloodstream.
After the first 5-7 days, it'll be "releasing" an amount in relationship to the dose given
It's also important to get tests on the PM cycle....Most cats go lower at night so it's important for his safety. Also, without getting tests on the PM cycle, you're missing half the data!
If you think of your spreadsheet like it's a puzzle, if you're only filling in the pieces on the left side, you don't know what's on the right side so you don't know if it's a puppy or a kitten next to that flower!...LOL