Black Box Taxes

Japanese taxes are due on March 15th, so this afternoon I got friendly with my pile of receipts and bank details. My head is now full of numbers and doubts, but my Heisei 15 tax forms are completed. Every filing I’ve submitted has been returned for corrections. I’m sure this year will be no different.
The Japanese tax system is a black box but I assume it follows the same basic pattern as the US. If you add up your earnings, subtract out any deductions, then multiply by your tax percentage, you’ll know how much tax you owe. But the details are a little hazy, so I don’t really try to understand. I simply follow along with the translated English instructions and do the math.
But even my indifferent attitude was pierced when I reached this calculation:
Total earnings from employment / 4 * 2.8 – 180,000 = employment income (please fill in line 6)
Huh?? Why? I guess it doesn’t matter. I have to say, I’m glad I have a calculator.