Taking control of the dynamic bitcoind tx fee
First: remind settxfee is not a absolute fee per transaction. It's a fee per kilobyte.

Example:


Alice sends 1BTC to Bob
Alice has 11 times 0.1 BTC in her wallet (she received 11 times 0.1 BTC from different persons)
Alice transaction will require all 11 x 0.1BTC inputs to create the 1BTC-to-Bob-transaction
Therefore the transaction is bigger (in terms of bytesize) then if she would hold 1 x 1.1BTC in her wallet.


Bitcoin-Core will only accept transaction to it's mempool which contains the minimum fee.