For example: you have 1BTC and you buy FCT for the amount of 0,2BTC. Therefore you have 0,8BTC and some FCT. So you insert the FCT equivalent in the app, ok, but I can't find the way to substract this amount of Bitcoin. How do you do this correctly?
If I understand this right, I think you mean entering an amount of Bitcoin, buying an alternative coin with some of that Bitcoin, then entering the amount of that alternative coin, but not subtracting it from your BTC balance. If that is the scenario you are speaking of, then you need to change the amount of bitcoin manually.
Thanks for the answer, what if you have bought BTC with different prices? I bought on different dates, you will never have a precise number. Or should you check the date of purchase of the altcoin and subtract the BTC amount from the last time you bought it?
Is there no button to say, bought X amount of BTC to buy Factom for example.
The amount of BTC (that you have) will be adjusted, in price, with the current price of Bitcoin.
I think you can just do that — it is tedious, but it's good for tracking your money.