You should know that the rule for calculating instalments from 1 April 2018 to 30 June 2018 no longer exists. It was part of the Income Tax Act 2007, but it has been removed. This rule was about tax credits that were paid in cash. The government decided to take this rule out of the law on 24 May 2021, but it actually stopped being used on 1 April 2021. This means that if you're looking for information about how to calculate instalments for this time period, you'll need to check for updated rules or talk to someone who knows about current tax laws.