Just below the estimator/calculator on the page I linked to at the tip it says:-
"If you use too much then over usage is carried over. Again, this is limited to the same as your tariff. If you use too much then you pay for the excess at £4.99/unit�.
The usage is all metered to the byte, and carry over from one bill to the next as well as excess usage charging is done in 0.01 unit steps. So you don't pay for a whole extra unit just for using part of it."
But the way you put the question implies you think you have a whole number of Off-peak units in your tariff to play with.
You don't! You have the total number of units you contracted for, and how you use them is nothing to do with how you estimated what you needed.
Assume you contract for 2 units on 20CN, and use 1.25GB in peak time because you are a workoholic overtime office worker. That leave you with 0.75 units.
Now assume you also use 60GB off-peak. That leaves you with 0.15 units.
Finally you also use 30GB night-time.
Your unused allowance is 0.12 units. You start the next billing period with 2.12 units.
This bit is wrong
"Your unused allowance is 0.12 units. You start the next billing period with 2.12 units."
As units are carried forward in 0.1 steps, the carry-forward is 0.10 and you start the next month with 2.10 units.
I also have
a more recent explanation which illustrates how the carry-forward of
over-usage works, (ie being able to eat into the following month's allowance).
Still based on the current rates though.