Don't just go by whatever percentage someone tells you - what really matters is figuring out how much you actually earned net after you subtract all your deductible expenses, because that's what the tax authority is going to charge you on.
A trick that works well is doing a quick projection every quarter: take your income for that period, subtract your expenses (equipment, software, internet, whatever you use for work), multiply that by your autonomous community's tax rate (they usually range from 19-45% depending on where you are), and that number is what you need to set aside. That way you won't come up short or have extra money sitting around unnecessarily, plus you get to see how the year is going without any nasty surprises when you file.