Can I estimate my own tax

I'm a freelancer, and I've been putting 25% aside for tax as I used to pay this amount of tax when I worked as an employee. Can I keep doing this as I am still earning the same gross amount each year?

Yes you can, have a look at our SARS income tax calculator and work out what the tax is on the amount earned and then use that as a base for the average tax on the income earned.