Well a starting point in the calculation is the loss to the family purse of Child Benefit that he attracted (if he is the "first" child then it is about £20 per week) and that he now no longer qualifies for. Then you need to calculate his travel costs, lunch money (unless Mrs G is packing him up sandwiches) and other expenses that he has to meet (Clothes, union fees, and the like)
When my older ones reached this stage we set out two columns of figures.
What they cost us and what they had to spend simply to go to work, buy their own clothes etc. These two are then added together and subtracted from the net income he will receive. You can then strike a balance about how much of the remainder he may have to spend as he wishes and how much he should save.
Roughly, as far as I recall, it worked out that 25% of his net pay was handed to his mother and he got the rest. This in no way covered our costs of him living at home but it was a start in making him accept that life is not free.
I never looked to make a profit but they have to accept that a contribution is expected. Living at home is still cheaper than getting a place of their own. I found it useful at this stage to demonstrate just how much housing them costs. If there are four in the house simply take 25% of all fixed costs i.e. those that have to be paid: Rent/Mortgage, Water Rates, Electricity, Gas, Council Tax etc etc. This is their share. It usually is a real eye opener to them. Of course you cannot expect them to pay 25% of all your housing costs and the really smartarsed ones will always argue that if you have a mortgage then you get the profit when you sell the house. My counter to that was that any profit would be their inheritance (That shut the argument down quickly) but a basic understanding of life's realities is no bad thing when they start earning.
