WC how often ?

hi there i am new to reeftanks



and i got told to do wc every 2 weeks as my tank is over 40 gallon its 47 about 216 l



i done a 15 % wc on wed



plus 10l for external filter so in total 35l is that enough or should i do about 40l



plus 10 for filter every 2 weeks ?



plus i have got some salt water left in my mixing bucket with a powehead to move it around how long will it be ok ? tia