You can change the paid to date for the pay, the number of tax periods over which the tax is to be averaged, or add a message to be printed on the pay advice by clicking on the More button on the Calculate Pay form.
The number of periods is set by the 'tax periods' pay item, but can be manually over-ridden here if required. A number entered here will average the tax of the whole pay slip.
The tax is calculated by dividing the gross pay by the number of periods entered, working out the tax on the result, and then multiplying the tax by the number of periods..
If you wish to have a message on multiple lines, press ctrl-enter to insert a line break.