Whenever I see posts like this honestly I think the "children" concerned haven't been parented in a way that makes them realise how the real world works, that prepares them for adulthood.
I'm a Lp to a now almost 18 year old who is working full time. She pays 1/4 of her take home to me in keep - which I have on occasion been flamed for! I am disabled and unable to work and on benefits. When she left school I lost a lot of income but she still needed to be housed, fed, clothed etc no way I could afford to do that on the benefits I receive which are only intended to provide me enough to live on.
I have talked with dd since she hit high school age about how much running a home costs honestly, not least because at times she's asked for items or to do things I simply can't afford and I've explained to her why.
She knows how much our rent & bills are. As its just the 2 of us tbh I could argue she should pay half of everything - which would be double what she does pay but I still want her to be able to enjoy her early years of earning.
Op you need to toughen up - seriously!
The pittance they're currently giving you I very much doubt covers even 1 of the things that they incur costs on! (Badly worded sorry) they need to at least cover the additional costs THEY incur - the energy they use, food, other groceries, wear & tear on furniture & appliances, the extra rent due to needing a larger property, ditto council tax plus the loss of single adult discount...
I hope to god you're not paying for their personal expenses eg phones?
As for your son smoking weed against your wishes (I am vehemently anti drugs, dd isn't into them thankfully but even if she were she wouldn't DARE Bringing them into our home let alone using them here!) you say you "can't" stop him doing that - of course you can! He stops using or storing in your home or he gets the fuck out! (What are the terms of your tenancy on smoking? Because many don't allow smoking in the property any more, certainly where I live!)
You are choosing to let them disrespect you & your home. You need to stop that right now!
Dd also keeps her room clean & tidy, does her own laundry, tidies any mess she makes elsewhere, cleaning the bathroom thoroughly once a week is her one job that benefits us both that I insist on, but off her own back she'll tidy things that she knows I'd struggle with and runs the Hoover round and is generally good if I ask her a favour.
It won't be you they hate, it will be getting called out on their freeloading, lazy arse attitudes! They've got it really bloody cushy at the moment!
You're not doing them any favours either. Again frequently seen on mn "kids" of all ages not expected to do chores - utterly unacceptable and unrealistic in my opinion.
They will find running their own home so much harder if they have to learn how to do so from scratch I'm guessing in their 20's!! And their housemates/partners/spouses definitely won't thank you!
Arrange a family meeting, possibly even a coffee outside the house to ensure suitable behaviour? (Shouldn't be necessary but I suspect it will be with these 2), have a plan in your mind and possibly written in black & white what the household outgoings are, perhaps with a separate breakdown of what they individually incur within those costs. I think pps suggestion of knowing what a local houseshare ave cost is too. So if they try and say you're asking for "too much" you can say to them "well it'd cost you £X extra to live elsewhere but of course that's your choice. Either live with me and pay £X (equal to or slightly less than a houseshare would cost them + the benefit of certain chores - but not all - done for them in a familiar safe home) or take your chances elsewhere! Also set out minimum ground rules (which is no different to what they'd have in a houseshare tbh) of they clean and tidy their own rooms/mess plus at least one chore that benefits everyone.
That is absolutely not asking too much and if they try to say it is don't listen to them!
"On money, I don’t think it’s fair to charge the going rate for a house share or a random percentage of their wages. For me, the following would be fair: Additional costs (e.g council tax), fair split of bills and food and in your case a portion of your rent as you’re in a bigger place than you need for their benefit." Usually what you're suggesting would actually work out to MORE than the amounts/percentages most are suggesting on thread.
Eg if I made dd pay as you suggest she'd need to pay at least £90 more than she does. That's around the amount of local houseshares BUT she'd have to buy groceries and personal items, plus bedding etc on top of that.
My siblings and I all paid 1/4 of our wages if we were living at home and working full time. A fraction/percentage is a fairer way of doing it especially if more than one "child" because it accounts for different wages, inflation etc.