Choose a split in advance. For example, half to the cushion or the costly debt, and half for something you will enjoy. The exact fractions can change. The habit is deciding before the money sits in the spending account for a week.
Check your tax withholding or tax code once a year as well. Paying far too much all year and waiting for a refund is an interest-free loan to the tax authority. Paying too little can mean a bill you did not plan. The right code is a saving of a different kind: fewer surprises.
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Write your windfall split on the same card as your savings goal, so the rule exists before the money does.