Going against the grain here, but I don't agree with dcs paying board money based on their earnings in their own home under any circumstance.
I was with dh (then dp) for 5 years before we managed to move out together, it was really hard work to go through university and save money working in jobs we hated, never seeing each other. All we wanted was a place of our own.
I got a job in a restaurant and hated it, but worked all hours to earn good money, while I ran my own business from the side. If my parents had decided to just raise my board because I could afford it, I would have been seriously upset, because our dream of having our own place would have been so much further away.
I contributed £60 a month- that paid for the broadband which I needed for my work. And I bought things like shower gels and food I had used for the whole family once it ran out.
I wouldn't be able to watch my DD work all hours and save every penny to get her independence and insist on taking some away from her each month so she 'learns how to budget', especially when we didn't need it, its daft.
I can understand if you have grown up dcs at home taking the piss, earning a good wage but not helping out around the house, making a mess, and using all the food and other things- I would insist a contribution there.
But charging dcs to live in their family home to teach them a lesson about life is stupid imo.
I would encourage your sons to put a bit away each week to save it, then they are in charge.