Why would I need to work out gross salary from a net (take-home) figure?
The most common reason is salary negotiation: you know you need a certain amount landing in your bank account each month to cover your budget, and you want to know what headline salary to ask for. It also comes up when comparing job offers quoted differently (one as a contract day rate, one as an annual salary), when freelancers or contractors are setting a target day rate, and when budgeting backwards from a target lifestyle to figure out what salary supports it.