Part of what makes it complicated is the way discounts are applied. The full, undiscounted charge first appears before all or part is subtracted. Neither the price per minute nor the number of minutes used is reported (unless you pay extra). To figure out how many minutes I am using, I had to sum the pre-discounted prices for all the different call types, and then find the exact cost per minute (¥42) for my plan from Docomo's website.
(¥3,800 + ¥260 + ¥40 + ¥420) / (¥21/30s)*2 = 108 minutes
As I was doing this, I realized that I am paying for "discount" services that cost me money, specifically the "yu-yu call discount" (ゆうゆうコール割引).
This allows you to register 5 domestic number for discounted calling - 30% off for Docomo numbers, 10% of other carriers. However, the service costs ¥180 per month. For the discount to be effective requires at least 15 minutes per month in the case of a Docomo number, 45 minutes for another carrier's number, or some combination of the two.
It turns out that I don't call enough to warrant the yu-yu plan. In the image above, I was only discounted ¥82. Over the past 12 months, I paid an additional ¥2,340 but only was only discounted ¥1000 in return. So, it actually cost me ¥1340. That's not a lot - ¥100 a month, but a hundred yen is a hundred yen. There was only one month when the math actually worked out in my favor. Guess how much money I saved.
Four. Whole. Yen.
Yeah, lemme go cancel that today, two days before the start of the August billing cycle.
The family plan on the other hand is definitely saving me money.