Using your logic, any method can be described as somehing else by simply using maths to make the subject something else.
What we presently do is take the chronological proportion of the relevant allowances and bands and use them to calculate the tax. The calculation doesn't work out the expected annual pay at all. You can tell that by looking at the tax tables.
What you have said is a bit like saying that instead of simply measuring the length of a car you actual incorporate the size of the earth into the calculations using multiplicating and divisionto arrive at the conclusion!
Relax, I'm being serious.