It is not necessary to know how to calculate your tax bill as there are a plethora of tax calculators which will perform the calculations for you. Of course the income or corporation tax return provides the calculation when you complete it but often that is well after the period when you should be setting aside the tax. It is useful to know your likely tax bill as you process through your accounting year so that you can set aside the funds to cover this.
Here are some useful calculators to use (click on the heading to follow the link):