Excellent question!
It is true that the only way we have to measure ketones is to measure what the body isn’t using, and it’s also true that ketones in the breath and urine are by definition “wasted,” but your body right now is still in the middle of changing over to using fat and ketone bodies, so don’t be discouraged. You are definitely producing ketones, which is the first step, so rejoice in that. Give it several weeks before you reach full fat-adaptation. It takes the body a while to readjust after years of burning glucose.
A lot of people find that their ketone measurements go down, once their body gets more efficient at using fat and ketones. Fortunately, you already know to expect that, so you won’t be panicking when it happens. In the meantime, just keep ketoing on, you’re doing fine!
I forgot to add that, if you keep measuring for any length of time, you may find your ketone levels going up and down at different times of day, and sometimes for no apparent reason. Don’t worry about it.