You don't necessarily use this to approximate expenses. You use it to determine the maximum tax credit you could receive for work-related expenses under this part of the tax code.
According to the examples listed on the page you linked (look immediately below the passage you quoted), you first look up the maximum employment amount by year. For 2012, it was $1,095.
Then, if your income falls below the maximum, you multiply the lowest tax rate by that amount. For example,
$800 was employment income reported in 2012. The income is below the
maximum amount of 1,095. $800 was claimed on line 363 in Schedule 1 as
Canada employment amount. The tax credit was $120 calculated by
multiplying the lowest federal tax rate of 15% by the amount ($800).
If your income was above the maximum amount, multiply the lowest marginal tax rate by the maximum amount, e.g.
$35,000 was employment income reported in 2012. The maximum amount of
$1,095 was claimed on line 363 in Schedule 1 as Canada employment
amount. The tax credit was $164.25 calculated by multiplying 15% by
The amount you calculate is the amount you're allowed to claim on line 363 of Schedule 1 of your federal tax return.