Yes in theory, total import minus rate01=rate02
As a double check go back to your last bill and check the numbers make sense for each rate and that you have them the right way round.
You should be able to read both rates, as a first try try reading it when it's actually switched to the lower rate somewhere after 22:30 or midnight?
What type of meter is it and who fitted it originally?
Also try pressing 6 a few times?