I lowered the minimum withdraw value to 0.002 BTC.
Where can I find detailed information about Bitcoin fees and how are they calculated?
I would like to minimize the fees and to decrease the minimum withdraw value even more.
It's possible to set the tx fee to 0 BTC, however this will cause VERY long times for confirmation. You can always use blockchain.info to set tx fees. The recommended for good confirmation speed is 0.0005 BTC for a tx fee, however even 1 satoshi is better than 0 BTC. Not that I recommend you do that, it's all up to preference ~
I was thinking of a tx fee of around 10% of the withdraw. This would be lower than the default tx fee of 0.0005 BTC, and higher than zero. It is possible to set what fee I want? This can be done only by using the blockchain.info wallet?
Thanks

It's easier to do it with the blockchain.info wallet to be honest.
Also, a tx fee of around 10% of the withdraw is a waste, if you're going to include a tx fee, make it 0.0005 or less. (although if the withdraw is <5 mBTC, I agree with the 10% thing).
There's no point tacking on a 0.2 BTC fee if someone withdraws 2 BTC, for instance.
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Lol.