I dont use hydroponics but I will say that you should stick to your guidelines, especially if you are new to it.

If you start to see browning on the leaves then start worrying. Can't you dilute the medium to bring it back to 7.5ml ?

Anyway, if you want to take the chance and leave the 20ml in there then go for it. If it survives the week the just don't add the full20ml the following week. Add15ml, and then go back to the guide lines.

Take care