The GST is not just 10% of sales. Tt is 10% of sales, less 10% of the purchases you make that have GST on them.
The easiest way and the way I do it for myself, is just see how much GST you end up paying this first quarter, and divide it by 13 and put that much aside each week. If you think your turnover is improving then maybe add a little bit more – you can also reassess this each quarter.
I have some clients who make sure that their bookkeeping is totally up to date, accurate and reconciled every week and do a tax summary report and transfer the exact amount off the report into their GST bank accounts. Personally, I think this is a little tedious and unnecessary but its up to you to choose what suits you best.