No because it won't be a 100% solution. You need to know the strength of the Sodium Metaborate solution.
Sodium Metaborate has never been made in Europe but a solution can be made using Borax and Sodium Hydroxide, for instance the Bellini Sodium Metaborate solution is 280g/litre, 28%.
So 100ml would contain 28g, for the equivalent of 20g you'd need to add 71.4ml, it doesn't need to be too accurate, it's there as a buffer.
Ian