I would be inclined to the 35% of take home pay. While it is wonderful for adult children to live at home with very little responsibility in terms of food, cleaning, washing, utilities etc., there is a significant impact on the household.
I would use the money to:
Do household improvements, all that wear and tear from the 20 odd years of kids, breakages, making do, buying low quality because that was all you could afford.
Improve the quality of food in the groceries.
Get the more luxurious shampoos, soaps etc.
Leave the heating on for longer than normal.
All those jobs you have been putting off for years because you were shelling out money for extras and education for the kids.
Spending this way improves the lives of all in the household. If you still have a mortgage pay it off faster.
And it focused the mind of the adult kid - do they want to continue to live at home or move out to spend their money as they wish. My adult kids are either still in education or living out of home so this is all theoretical on my side. I might think differently when it actually happens. But I did have a friend who paid 50 % of take home pay to live at home.
Sound advice. There have been multiple posts on AAM by parents solving all their children's problems and bankrolling their lifestyles, storing up future practical and mental health problems for them in the process.