It would be nice to have a page about this on the wiki, because the explanation is not that complicated; it just seems that way when people have different underlying assumptions about how it works.
If you just want to know how much capital gains tax you will pay when you sell some funds?
On Rakuten you'll see a screen like this (on the buy/sell page and the holdings listing page):
時価評価額/評価損益 is your profit/loss. It's shown in yen by default; click on the % button to show as a percentage. In this case, the fund has gains (profit) of 23.37%.
Capital gains tax is 20.315% of gains. So if I sell some of this fund my net tax rate will be (23.37 x 0.20315) = 4.75%.
If I sell Y1,000,000 worth of this fund, I will pay about Y47,500 in tax and the rest, about Y952,500, will be deposited into my cash account.
In this case the fund has appreciated a lot more and my net tax rate would be (187.1 x 0.20315) = 38.01% so I would be paying about Y380,100 and netting Y619,900 on a Y1,000,000 withdrawal.
That's how capital gains tax works. The 時価評価額/評価損益 value is all you need to figure out how much tax you'll be paying.*
Hopefully this didn't come off as smug or anything; I'm just trying to be straightforward. Nobody is born knowing how capital gains taxes work!
Obviously, someone please correct me if there's any error in this post!
* edit In case it's not clear what exactly 時価評価額/評価損益 indicates: This is simply the total amount you've paid into the fund (known as "cost basis"), compared to its current market value. That's assuming you haven't yet sold off any of the fund; there will be extra steps in calculating the cost basis if you have previously sold some shares, but hopefully you understand the principle.
